42 Isospondyli 



vertebral centra are feebly developed, and the dorsal fin short 

 and posterior. In' Leptolepis the anal is short and placed 

 behind the dorsal. There are many species, mostly from the 

 Triassic and lithographic shales of Europe, one being found 

 in the Cretaceous. Leptolepis coryphccnoides and Leptolepis 

 dubius are among the more common species. Mthalion (knorri) 

 differs in the form of the jaws. In Thrissops the anal fin is 

 long and opposite the dorsal. Thrissops salmonea is found 

 in the lithographic stone; Thrissops exigua in the Cretaceous. 

 In all these early forms there is a hard casque over the brain- 

 cavity, as in the living types, Amia and Osteoglossum. 



The Elopidae. The family of Elopidce contains large fishes 

 herring-like in form and structure, but having a flat membrane- 



FIG. 28. Ten-pounder, Elops saurus L. An ally of the earliest bony fishes. 



Virginia. 



bone or gular plate between the branches of the lower jaw, as 

 in the Ganoid genus Amia. The living species are few, abound- 

 ing in the tropical seas, important for their size and numbers, 



FIG. 29. A primitive Herring-like fish, Holcolepis lewesiensis , Mantell, restored. 

 Family Elopidce. English Chalk. (After Woodward.) 



though not valued as food-fishes save to those who, like the 

 Hawaiians and Japanese, eat fishes raw. These people prefer 



