In the surviving Crossopterygii (e.g. Polypterus[[199]]) the chondro-cranium is complete in the ethmoidal and post-orbital regions, except where it has been partially replaced by cartilage bones, but in the inter-orbital region the continuity of the roof is interrupted by a large fontanelle, which is only closed by the investing frontal bones (Fig. 132, C). There is also a large basi-cranial fontanelle in the sphenethmoid, closed, however, by the underlying parasphenoid. A large "occipital" bone continuously ossifies in the occipital cartilage and completely surrounds the foramen magnum.
Fig. 132.—A, side view of the skull of Polypterus; B, dorsal view, showing the chief dermal bones; C, similar view of the chondro-cranium after the removal of the dermal bones. An, Angular; Ar, articular; D, dentary; E, mesethmoid; f.m, foramen magnum; Fr, frontal; l.e, lateral ethmoid; Mx, maxilla; Na, Na′, nasal and accessory nasal bones; occ, occipital; ol, nasal aperture; Op, operculum; op.o, opisthotic; O.t, os terminate; Pa, parietal; Pm.x, premaxilla; P.t, post-temporal; Ptf, post-frontal; Qu, quadrate; S.b, S.b′, circum-orbital ossicles; S.Op, sub-operculum; Sp, splenial; sp.eth, sphenethmoid; sp.o, sphenotic; Spr, spiracular ossicles, between which is the spiracle; S.t, supra-temporals; Y, cheek-plate (pre-operculum); Y′, Y″, smaller cheek-plates; z, z, z, z, post-spiracular ossicles; z′, z′, prespiracular ossicles. In C the cartilage is dotted. (From Traquair.)
Prootics and pterotics are absent, and the opisthotics seem to be confluent with their respective epiotics. The floor and side walls of the inter-orbital section of the cranium are formed by a remarkable "sphenethmoid" bone which occupies the position of the paired ali- and orbito-sphenoids in other bony Fishes; and in one species, P. lapradei,[[200]] it forms in front distinct tubular investments round the olfactory nerves. In many respects this bone is singularly like the sphenethmoid bone of the Frog and other tailless Amphibia. A median ethmoid as well as lateral ethmoids are present. In addition to the ordinary dermal bones which invest the cranial roof there is a transverse row of supra-temporal plates crossing the cranial roof behind the paired parietals (Fig. 132, A). Fringing the outer margins of the frontals and parietals a row of pre- and post-spiracular ossicles extends nearly to the orbits, and between two of them, which form a valve, is the spiracular aperture itself. There is a dentigerous splenial on the inner surface of the lower jaw. The hyoid arch has no separate symplectic bone. An operculum and a suboperculum are present, but no inter-operculum; and unless the hinder part of the large cheek-plate, which is traversed by the mandibulo-hyoid sensory canal, represents a pre-operculum, the latter is wanting. Branchiostegal rays are absent, but there is a single pair of large jugular plates.
Very little is certainly known about the cranial cartilage-bones in the fossil members of the group, but the investing dermal bones, which bear a general resemblance to those of Polypterus, are often somewhat more numerous, and they form a very complete dermal armature for the entire head. There is a very complete ring of circum-orbital bones, and very often a ring of sclerotic plates. Two large cheek-plates are often present. Nothing comparable to pre- and post-spiracular ossicles is known, but squamosal and supra-temporals can often be identified. To the ordinary bones of the lower jaw there may be added a series of infra-dentary plates, and besides the paired principal jugular plates there may also be present a small anterior median plate and a series of small lateral jugular plates on each side, as in the Carboniferous Rhizodopsis (Fig. 274). Most of the superficial dermal bones, both in the living and extinct Crossopterygii, are invested externally by a granulated or rugose layer of enamel-like ganoin.
In the Holostei, and especially in Amia, the skull approximates more closely to the normal Teleostean type as represented by the Salmon's skull. In Amia[[201]] all the occipital cartilage-bones are present—a basi-occipital, two exoccipitals, and a supra-occipital; and, except for the absence of a pterotic, the periotic series of bones is also complete. Paired ali- and orbito-sphenoids form the lateral walls of the inter-orbital portion of the cranial cavity. Above, the complete cartilaginous roof of the cranial cavity is invested by a shield of suturally united and ganoin-covered dermal plates. The hyomandibular element has a symplectic bone at its distal extremity. There is a complete series of opercular bones, and the branchiostegal rays are numerous. A single median jugular plate is present. The lower jaw has on each side five dentigerous splenial bones in addition to dentary and angular bones, while cartilage-bones are represented by articular and mento-Meckelian elements. In its essential structure the skull of Lepidosteus[[202]] resembles that of Amia, but it has obviously undergone much specialisation. In some species (e.g. L. osseus) its appearance is greatly modified by the exceptional length and tapering shape of the beak, due to the elongation of that part of the skull which lies between the orbital and nasal regions; but in L. platycephalus the reduced length and greater width of the beak, combined with its somewhat flattened condition, impart an almost Crocodilian aspect to the head. Amongst other points of difference it may be mentioned that in Lepidosteus the continuity of the chondro-cranial roof is interrupted by a large superior fontanelle. There is no supra-occipital, and there are no lateral ethmoids, at all events in the usual position. The inter-orbital portion of the cranial cavity is largely obliterated by the formation of an inter-orbital septum, consisting of a thin vertical plate of bone, which either represents a pair of fused orbito-sphenoids or a pair of similarly modified lateral ethmoids. In addition to the ordinary investing dermal bones, including circum-orbitals, squamosal, and supra-temporals, there are numerous scale-like ossicles which take the place of the cheek-plates of Polypterus. The maxillae are segmented into numerous dentigerous bones fringing the margins of the upper jaw. The lower jaw has no mento-Meckelian bones, but there is a very complete series of dermal elements, including dentary, coronary, splenial, angular, and supra-angular bones in addition to an articular cartilage-bone. One of the most remarkable features in the skull of Lepidosteus is the existence of a secondary articulation between the metapterygoid bones and a pair of transversely elongated condyles formed on each side by a lateral outgrowth from the parasphenoid and alisphenoid bones. By a horizontal sliding movement of the former on the latter, provision is made for the lateral expansion and contraction of the walls of the oral cavity and the separation and approximation of the lateral halves of the upper jaw.[[203]]
The generality of Teleosts[[204]] more or less closely agree with Amia in the main features of their cranial structure. There are, however, certain minor features which are characteristic if not always distinctive of the group. As a rule, to which, nevertheless, there are notable exceptions, there is little of the primary cartilaginous cranium in the adult, nearly the whole of it having become absorbed or converted into cartilage-bones. A supraoccipital is invariably present, and usually a mesethmoid and a basisphenoid. An additional bone is added to the periotic series, viz. a pterotic. Supra-temporal bones and jugular plates are always absent, and it may be doubted if mento-Meckelian bones and dentigerous splenials are ever developed in the lower jaw. Within the group itself the skull exhibits many notable modifications, of which only a few can here be mentioned. The shape, size, and character of the mouth and jaws, the extent to which they can be protruded and retracted, and the nature of the dentition, are the source of many characteristic modifications in the structure and appearance of the fore-part of the skull, and these again largely depend upon differences of habit and food. A protrusible mouth, or a mouth which is projected forwards, is usually associated with a suspensorium (hyomandibular) of considerable length, and so greatly inclined forwards as to make a more or less acute angle with the forepart of the cranium.
The presence or absence of an inter-orbital septum is also a feature in which considerable variation occurs. In some Teleosts there is no septum, and the cranial cavity is prolonged forwards between the orbits, where its lateral walls are formed by well-developed, paired ali- and orbito-sphenoid bones, as, for example, in the Carp and other Cyprinidae. In others the fusion of the cranial walls is accompanied by the median union of the orbito-sphenoids, so that a partly bony and partly cartilaginous inter-orbital septum is found, and the cranial cavity becomes largely obliterated in this region, as in the Salmon; or the orbito-sphenoids may be non-existent, the cartilage may undergo absorption, and the inter-orbital septum may become reduced to a vertical fibrous sheath extending between the frontals above and the parasphenoid below, as is the case in the Cod (Gadus).
An interesting modification of certain of the bones of the primary and secondary upper jaw occurs in the Siluridae. In these Fishes the maxillae are very small and edentulous, and serve no other purpose than forming basal supports for the maxillary barbels, while the rod-like palatine bone, losing its connexion with the pterygoid portion of the primitive upper jaw, but retaining its articulation with the lateral ethmoid, serves to support the maxilla, and at the same time receives the insertion of the muscles by which the barbel is moved in various directions.
In the Plectognathi the premaxillae are co-ossified with the maxillae. Many other interesting cranial modifications occur in Teleosts, and to some of them reference is made in subsequent chapters.