[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.