FOOTNOTES:
[157] "The name Arthrodira as given to Coccosteans, as distinguished from the Antiarcha, is not altogether a satisfactory one, since at least from the time of Pander the head of Pterichthys (Asterolepis) is known to be articulated with the armoring of the trunk in a way closely resembling that of Coccosteus. This term may, however, be retained as a convenient one for the order of Coccosteans, in which, together with other differentiating features, this structure is prominently evolved. A renewed examination of the subject has caused me to incline strongly to the belief, as above expressed, that Pterichthys and Coccosteans are not as widely separated in phylogeny as Smith Woodward, for example, has maintained. But, as far as present evidence goes, they appear to me certainly as distinct as fishes are from amphibia, or as reptiles are from birds or from mammals." (Dean.)
The name Placodermi used by McCoy in 1848 was applied to the Ostracophores as well as to the Arthrodires. Hay revives it as the name of a superorder to include the Antiarcha and the Arthrodira, the former being detached from the Ostracophores. This superorder is equivalent to the subclass Azygostei of Hay.
[158] Memoirs New York Academy of Sciences, 1901.
[159] It will be recalled that there is no ground for concluding that the "mandibular rami" possessed an endoskeletal core, and were comparable, therefore, to the somewhat mobile jaws of Elasmobranchs. On the other hand, there is the strongest evidence that they are entirely comparable to adjacent dermal plates. Histologically they are identical, and in certain cases their exposed surfaces bear the same tuberculation.
[160] The similarity of Macropetalichthys to Dinichthyids in the general matter of the dermal plates is so complete that I have had no hesitation in associating it with the Arthrognaths. (Cf. Eastman.) The circumstance that its "jaws" have not yet been found has to a large degree been due to the lack of energy on the part of local collectors. In the corniferous quarries near Delaware, Ohio, this fossil is stated to be relatively abundant.
[161] It is by no means impossible that there may ultimately be found pectoral elements to correspond in a general way with the paddle-like "spines" of the Antiarcha.
[162] The group Placodermi, created by McCoy (1848) as a "family" for the reception of Coccosteus and Pterichthys might then be justly elevated to rank as a class, superseding the Ostracophori of Cope (1891). The latter group might, however, be retained as a subclass, and include the Heterostraci and Osteostraci as ordinal divisions.
[CHAPTER XXXIV]
THE CROSSOPTERYGII
Class Teleostomi.—We may unite the remaining groups of fishes into a single class, for which the name Teleostomi (τέλεος, true; στόμα, mouth), proposed by Bonaparte in 1838, may be retained. The fishes of this class are characterized by the presence of a suspensorium to the mandible, by the existence of membrane-bones (opercles, suborbitals, etc.) on the head, by a single gill-opening leading to gill-arches bearing filamentous gills, and by the absence of claspers on the ventral fins. The skeleton is at least partly ossified in all the Teleostomi. More important as a primary character, distinguishing these fishes from the sharks, is the presence typically and primitively of the air-bladder. This is at first a lung, arising as a diverticulum from the ventral side of the œsophagus, but in later forms it becomes dorsal and is, by degrees, degraded into a swim-bladder, and in very many forms it is altogether lost with age.
This group comprises the vast majority of recent fishes, as well as a large percentage of those known only as fossils. In these the condition of the lung can be only guessed.
The Teleostomi are doubtless derived from sharks, their relationship being possibly nearest to the Ichthyotomi or to the primitive Chimæras. The Dipnoans among Teleostomi retain the shark-like condition of the upper jaw, made of palatal elements, which may be, as in the Chimæra, fused with the cranium. In the lower forms also the primitive diphycercal or protocercal form of tail is retained, as also the archipterygium or jointed axis of the paired fins, fringed with rays on one or both sides.
We may divide the Teleostomes, or true fishes, into three subclasses: the Crossopterygii, or fringe-fins; the Dipneusti, or lung-fishes; Actinopteri, or ray-fins, including the Ganoidei and the Teleostei, or bony fishes. Of these many recent writers are disposed to consider the Crossopterygii as most primitive, and to derive from it by separate lines each of the remaining subclasses, as well as the higher vertebrates. The Ganoidei and Teleostei (constituting the Actinopteri) are very closely related, the ancient group passing by almost imperceptible degrees into the modern group of bony fishes.
Subclass Crossopterygii.—The earliest Teleostomes known belong to the subclass or group called after Huxley, Crossopterygii (κρόσσος, fringe; πτερύξ, fin). A prominent character of the group lies in the retention of the jointed pectoral fin or archipterygium, its axis fringed by a series of soft rays. This character it shares with the Ichthyotomi among sharks, and with the Dipneusti. From the latter it differs in the hyostylic cranium, the lower jaw being suspended from the hyomandibular, and by the presence of distinct premaxillary and maxillary elements in the upper jaw. In these characters it agrees with the ordinary fishes. In the living Crossopterygians the air-bladder is lung-like, attached by a duct to the ventral side of the œsophagus. The lung-sac, though specialized in structure, is simple, not cellular as in the Dipnoans. The skeleton is more or less perfectly ossified. Outside the cartilaginous skull is a bony coat of mail. The skin is covered with firm scales or bony plates, the tail is diphycercal, straight, and ending in a point, the shoulder-girdle attached to the cranium is cartilaginous but overlaid with bony plates, and the branchiostegals are represented by a pair of gular plates.
In the single family represented among living fishes the heart has a muscular arterial bulb with many series of valves on its inner edge, and the large air-bladder is divided into two lobes, having the functions of a lung, though not cellular as in the lung-fishes.
The fossil types are very closely allied to the lung-fishes, and the two groups have no doubt a common origin in Silurian times. It is now usually considered that the Crossopterygian is more primitive than the lung-fish, though at the same time more nearly related to the Ganoids, and through them to the ordinary fishes.
Origin of Amphibians.—From the primitive Crossopterygii the step to the ancestral Amphibia, which are likewise mailed and semi-aquatic, seems a very short one. It is true that most writers until recently have regarded certain Dipneustans as the Dipteridæ as representing the parents of the Amphibians. But the weight of recent authority, Gill, Pollard, Boulenger, Dollo, and others, seems to place the point of separation of the higher vertebrates with the Crossopterygians, and to regard the lobate pectoral member of Polypterus as a possible source of the five-fingered arm of the frog. This view is still, however, extremely hypothetical and there is still much to be said in favor of the theory of the origin of Amphibia from Dipnoans and in favor of the view that the Dipnoans are also ancestors of the Crossopterygians.
Fig. 370.—Shoulder-girdle of Polypterus bichir. Specimen from the White Nile.
In the true Amphibians the lungs are better developed than in the Crossopterygian or Dipnoan, although the lungs are finally lost in certain salamanders which breathe through epithelial cells. The gills lose, among the Amphibia, their primitive importance, although in Proteus anguineus of Austria and Necturus maculosus, the American "mud-puppy" or water-dog, these persist through life. The archipterygium, or primitive fin, gives place to the chiropterygium, or fingered arm. In this the basal segment of the archipterygium gives place to the humerus, the diverging segments seen in the most specialized type of archipterygium (Polypterus) become perhaps radius and ulna, the intermediate quadrate mass of cartilage possibly becoming carpal bones, and from these spring the joints called metacarpals and phalanges. In the Amphibians and all higher forms the shoulder-girdle retains its primitive insertion at a distance from the head, and the posterior limbs remain abdominal.
The Amphibians are therefore primarily fishes with fingers and toes instead of the fringe-fins of their ancestors. Their relations are really with the fishes, as indicated by Huxley, who unites the amphibians and fishes in a primary group, Ichthyopsida, while reptiles and birds form the contrasting group of Sauropsida.
Fig. 371.—Arm of a frog.
The reptiles differ from the Amphibians through acceleration of development, passing through the gill-bearing stages within the egg. The birds bear feathers instead of scales, and the mammals nourish their young by means of glandular secretions. Through a reptile-amphibian ancestry the birds and mammals may trace back their descent from palæozoic Crossopterygians. In the very young embryo of all higher vertebrates traces of double-breathing persist in all species, in the form of rudimentary gill-slits.
The Fins of Crossopterygians.—Dollo and Boulenger regard the heterocercal tail as a primitive form, the diphycercal form being a result of degradation, connected with its less extensive use as an organ of propulsion. Most writers who adopt the theory of Gegenbaur that the archipterygium is the primitive form of the pectoral fin are likely, however, to consider the diphycercal tail found associated with it in the Ichthyotomi, Dipneusti, Crossopterygii as the more primitive form of the tail. From this form the heterocercal tail of the higher sharks and Ganoids may be derived, this giving way in the process of development to the imperfectly homocercal tail of the salmon, the homocercal tail of the perch, and the isocercal tail of the codfish and its allies, the gephyrocercal and the leptocercal tail, tapering or whip-like, representing various stages of degeneration. Boulenger draws a distinction between the protocercal tail, the one primitively straight, and the diphycercal tail modified, like the homocercal tail, from an heterocercal ancestry.
Fig. 372.—Polypterus congicus, a Crossopterygian fish from the Congo River. Young, with external gills. (After Boulenger.)
Orders of Crossopterygians.—Cope and Woodward divide the Crossopterygia into four orders or suborders, Haplistia, Rhipidistia, Actinistia, and Cladistia. To the latter belong the existing species, or the family of Polypteridæ, alone. Boulenger unites the three extinct orders into one, which he calls Osteolepida. In all three of these the pectorals are narrow with a single basal bone, and the nostrils, as in the Dipneustans, are below the snout. The differences are apparently such as to justify Cope's division into three orders.
Haplistia.—In the Haplistia the notochord is persistent, and the basal bones of dorsal and anal fins are in regular series, much fewer in number than the fin-rays. The single family Tarrassiidæ is represented by Tarrasius problematicus, found by Traquair in Scotland. This is regarded as the lowest of the Crossopterygians, a small fish of the Lower Carboniferous, the head mailed, the body with small bony scales.
Rhipidistia.—In the Rhipidistia the basal bones of the median fins ("axonosts and baseosts") are found in a single piece, not separate as in the Haplistia. Four families are recognized, Holoptychiidæ, Megalichthyidæ, Osteolepidæ, and Onychodontidæ, the first of these being considered as the nearest approach of the Crossopterygians to the Dipnoans.
The Holoptychiidæ have the pectoral fins acute, the scales cycloid, enameled, and the teeth very complex. Holoptychius nobilissimus is a very large fish from the Devonian. Glyptolepis leptopterus from the Lower Devonian is also a notable species. Dendrodus from the Devonian is known from detached teeth.
Fig. 373.—Basal bone of dorsal fin, Holoptychius leptopterus (Agassiz). (After Woodward.)
In the Ordovician rocks of Cañon City, Colorado, Dr. Walcott finds numerous bony scales with folded surfaces and stellate ornamentation, and which he refers with some doubt to a Crossopterygian fish of the family Holoptychiidæ. This fish he names Eriptychius americanus. If this identification proves correct, it will carry back the appearance of Crossopterygian fishes, the earliest of the Teleostome forms, to the beginning of the Silurian, these Cañon City shales being the oldest rocks in which remains of fishes are known to occur. In the same rocks are found plates of Ostracophores and other fragments still more doubtful. It is certain that our records in palæontology fall far short of disclosing the earliest sharks, as well as the earliest remains of Ostracophores, Arthrodires, or even Ganoids.
Megalichthyidæ.—The Megalichthyidæ (wrongly called "Rhizodontidæ") have the pectoral fins obtuse, the teeth relatively simple, and the scales cycloid, enameled. There are numerous species in the Carboniferous rocks, largely known from fragments or from teeth. Megalichthys, Strepsodus, Rhizodopsis, Gyroptychius, Tristichopterus, Eusthenopteron, Cricodus, and Sauripterus are the genera; Rhizodopsis sauroides from the coal-measures of England being the best-known species.
The Osteolepidæ differ from the Megalichthyidæ mainly in the presence of enameled rhomboid scales, as in Polypterus and Lepisosteus. In Glyptopomus these scales are sculptured, in the others smooth. In Osteolepis, Thursius, Diplopterus, and Glyptopomus a pineal foramen is present on the top of the head. This is wanting in Parabatrachus (Megalichthys of authors). In Osteolepis, Thursius, and Parabatrachus the tail is heterocercal, while in Diplopterus and Glyptopomus it is diphycercal. Osteolepis macrolepidotus and numerous other species occur in the Lower Devonian. Diplopterus agassizii is common in the same horizon. Megalichthys hibberti is found in the coal-measures, and Glyptopomus minimus in the Upper Devonian. Palæosteus is another genus recently described.
Fig. 374.—Gyroptychius microlepidotus Agassiz. Devonian. Family Megalichthyidæ. (After Pander.)
The Onychodontidæ are known from a few fragments of Onychodus sigmoides from the Lower Devonian of Ohio and Onychodus anglicus from England.
Fig. 375.—Cœlacanthus elegans Newberry. From the Ohio Carboniferous, showing air-bladder. (After Dean.)
Order Actinistia.—In the Actinistia there is a single fin-ray to each basal bone, the axonosts of each ray fused in a single piece. The notochord is persistent, causing the back-bone in fossils to appear hollow, the cartilaginous material leaving no trace in the rocks. The genera and species are numerous, ranging from the Subcarboniferous to the Upper Cretaceous, many of them belonging to Cœlacanthus, the chief genus of the single family Cœlacanthidæ. In Cœlacanthus the fin-rays are without denticles. Cœlacanthus granulatus is found in the European Permian. Cœlacanthus elegans of the coal-measures is found in America also. In Undina the anterior fin-rays are marked with tubercles. Undina penicillata and Undina gulo from the Triassic are well-preserved species. In Macropoma (lewesiensis) the fin-rays are robust, long, and little articulated. Other genera are Heptanema, Coccoderma, Libys, Diplurus, and Graphiurus. Diplurus longicaudatus was found by Newberry in the Triassic of New Jersey and Connecticut.
Fig. 376.—Undina gulo Egerton; Lias. Family Cœlacanthidæ. (After Woodward.)
Order Cladistia.—In the Cladistia the axis of the pectoral limb is fan-shaped, made of two diversified bones joined by cartilage. The notochord is restricted and replaced by ossified vertebræ. The axonosts of the dorsal and anal are in regular series, each bearing a fin-ray. The order contains the single family Polypteridæ. In this group the pectoral fin is formed differently from that of the other Crossopterygians, being broad, its base of two diverging bones with cartilage between. This structure, more specialized than in any other of the Crossopterygians or Dipneusti, has been regarded by Gill and others, as above stated, as the origin of the fingered hand (chiropterygium) of the frogs and higher vertebrates. The base of the diverging bones has been identified as the antecedent of the humerus, the bones themselves as radius and ulna, while the intervening non-ossified cartilage breaks up into carpal bones, from which metacarpals and digits ultimately diverge. This hypothesis is open to considerable doubt. The nostrils, as in true fishes, are superior. The body in these fishes is covered with rhombic enameled scales, as in the garpike; the head is similarly mailed, but, in distinction from the garpike, the anterior rays of the dorsal are developed as isolated spines.
The young have a bushy external gill with a broad scaly base. The air-bladder is double, not cellular, with a large air-duct joining the ventral surface of the œsophagus. The intestine has a spiral valve.
The cranium, according to Boulenger ("Poissons du Bassin du Congo," p. 11), is remarkable for its generalized form, this character forming a trait of union between the Ganoids and the primitive Amphibia or Stegocephali. Without considering Polypterus, it is not possible to interpret the homologies of the cranium of the amphibians and the sharks.
The jaws are similar to those of the vertebrates higher than fishes. Tooth-bearing premaxillaries and dentaries are solidly joined at the front of the cranium, and united by a suture to the toothed maxillaries which form most of the edge of the mouth. Each half of the lower jaw consists of four elements, covering Meckel's cartilage, which is ossified at the symphysis. These are the articular, angular, dentary, and splenial (coronoid). Most of these bones are armed with teeth. The palato-suspensory consists of hyomandibular, quadrate, ectopterygoid, entopterygoid, metapterygoid, and palatine elements, the pterygoid elements bearing teeth. In Erpetoichthys only the opercle is distinct among the gill-covers. In Polypterus there is a subopercle also; the suborbital chain is represented by two small bones.
The gill-arches are four, but without lower pharyngeals. The teeth are conic and pointed, and in structure, according to Agassiz, they differ largely from those of bony fishes, approaching the teeth of reptiles.
Fig. 377.—Lower jaw of Polypterus bichir, from below.
The external gill of the young, first discovered by Steindachner in 1869, consists of a fleshy axis bordered above and below by secondary branches, themselves fringed. In form and structure this resembles the external gills of amphibians. It is inserted, not on the gill-arches, but on the hyoid arch. Its origin is from the external skin. It can therefore not be compared morphologically with the gills of other fishes, nor with the pseudobranchiæ, but rather with the external gills of larval sharks. The vertebræ are very numerous and biconcave as in ordinary fishes. Each of the peculiar dorsal spines is primitively a single spine, not a finlet of several pieces, as some have suggested. The enameled, rhomboid scales are in movable oblique whorls, each scale interlocked with its neighbors.
Fig. 378.—Polypterus congicus, a Crossopterygian fish from the Congo River. Young, with external gills. (After Boulenger.)
Fig. 379.—Polypterus delhezi Boulenger. Congo River.
The shoulder-girdle, suspended from the cranium by post-temporal and supraclavicle, is covered by bony plates. To the small hypercoracoid and hypocoracoid the pectoral fin is attached. Its basal bones may be compared to those of the sharks, mesopterygium, propterygium, and metapterygium, which may with less certainty be again called humerus, radius, and ulna. These are covered by flesh and by small imbricated scales. The air-bladder resembles the lungs of terrestrial vertebrates. It consists of two cylindrical sacs, that on the right the longer, then uniting in front to form a short tube, which enters the œsophagus from below with a slit-like glottis. Unlike the lung of the Dipneusti, this air-bladder is not cellular, and it receives only arterial blood. Its function is to assist the respiration by gills without replacing it.
The Polypteridæ.—All the Polypteridæ are natives of Africa. Two genera are known, no species having been found fossil. Of Polypterus, Boulenger, the latest authority, recognizes nine species: six in the Congo, Polypterus congicus, P. delhezi, P. ornatipinnis, P. weeksi, P. palmas, and P. retropinnis; one, P. lapradei, in the Niger; and two in the Nile, Polypterus bichir and P. endlicheri. Of these the only one known until very recently was Polypterus bichir of the Nile.
These fishes in many respects resemble the garpike in habits. They live close on the mud in the bottom of sluggish waters, moving the pectorals fan-fashion. If the water is foul, they rise to the surface to gulp air, a part of which escapes through the gill-openings, after which they descend like a flash. In the breeding season these fishes are very active, depositing their eggs in districts flooded in the spring. The eggs are very numerous, grass-green, and of the size of eggs of millet. The flesh is excellent as food.
Fig. 380.—Erpetoichthys calabaricus Smith. Senegambia. (After Dean.)
The genus Erpetoichthys contains a single species, Erpetoichthys calabaricus,[163] found also in the Senegal and Congo. This species is very slender, almost eel-like, extremely agile, and, as usual in wriggling or undulating fishes, it has lost its ventral fin. It lives in shallow waters among interlaced roots of palms. When disturbed it swims like a snake.