ACTINOPTERYQII. !)"> 



;m<l the abdominal region of EwryoormtU (fig. 77 A, B, p. 108). 

 Frequently the hypocentral wedge fuses with the adjacent 

 pleurocentral wedge to form a complete ring (seen in some 

 Wealden species of Lepidotus). Sometimes in the caudal 

 region (e.g., Eurycormus, fig. 77 c) the pleurocentra and 

 hypocentra are completed alternating rings (the embolomerous 

 type of ossification in the notochordal sheath). In the higher 

 forms, each composite ring becomes thickened and constricts 

 the notochord, resulting in a typical vertebral centrum ; while 

 the alternating pleurocentral and hypocentral rings of the 

 caudal region become discs like those of Amia. The mandible 

 is always tolerably complex, exhibiting splenial and coronoid 

 (surangular) elements. 



The Protospondyli are the dominant fishes of the Jurassic 

 Period, and, like the Chondrostei of the Carboniferous and 

 Permian, they appear to become specialized in two directions. 

 One series, beginning with Acentrophorus and Semionotus, 

 seems to terminate in deep-bodied fishes with grinding teeth 

 (Pycnodontidse) ; the other, beginning with Eugnathus, tends 

 towards agile predaceous fishes with large conical teeth (Pachy- 

 cormidae). Both types are already well represented in Triassic, 

 Rhretic, and Liassic formations. 



The Permian genus Acentrophorus, already mentioned, is 

 the earliest representative of the great family of Semionotidae. 

 These are fishes with a small mouth and for ward ly-directed 

 suspensorium, the teeth when specialized becoming tritoral. 

 There are well-developed cheek-plates, and the opercular 

 apparatus is complete. There are long ribs and the arches 

 of the vertebral axis are nearly always well ossified ; but the 

 vertebral centra are never more than rings. All the fins exhibit 

 large fulcra and robust, more or less widely-spaced rays ; while 

 the dorsal and anal fins are much extended only in the deep- 

 bodied genera. The scales are at least in part rhombic and 

 united by a peg-and-socket articulation ; but they are some- 

 times thin, while in one case they are cycloidal and imbricating 

 on the caudal region. 



Semionotus. A fusiform fish with ossified ribs, entirely covered with 

 rhombic scales, which are not much deepened on the flank ; the dorsal 



