414 STUDIES FOR STUDENTS 



The body was protected by large enameled scales, and the 

 head terminated in a long and sharp rostrum. 



Palceoniscus is one the most important of the fossil CJiondros- 

 teans. It has a remarkable time range extending from the Palae- 

 ozoic to the Mesozoic and developed a very large number of 

 species. It is supposed to be the form that stands nearest to 

 the ancestral type connecting the modern garpikes and the 

 sturgeons. 



These forms seem to have culminated in the modern stur- 

 geons, in which the scales have almost entirely disappeared. 

 In Acipe?iser a few rows of large dermal, enameled plates is all 

 that is left, while in such forms as Polyodon, the spoon-billed 

 catfish, the skin is entirely naked. 



A second group of the Chondrosteans, developed mostly in 

 the Mesozoic, had much more the appearance of the pure bony 

 fish. The scales were small and rounded, and the fins are simi- 

 lar in shape and arrangement to some of the Teleocephali. 

 The bones are calcified and the tail is nearly homocercal, but 

 the vertebras are still unossified and the notochord is prominent. 

 The modern Amia, dogfish, is a surviving member of the group. 

 Among the fossil forms are Caturus, Megalurus, and Leptolepis, 

 from the Jurassic of Solenhofen. The first two of these are 

 important in showing the formation of the vertebras from the 

 gradual development of bone in the centrum starting from the 

 bases of the upper and the lower arches. In the first two of 

 these forms the base of each arch is joined to a half-moon 

 shaped element that is broad at the base and comes to a point 

 at about the center of the centrum, the two together forming a 

 ring that surrounds the notochord. In Eurycormus, from about 

 the same horizon as the last, the same condition prevails in the 

 vertebras of the dorsal region, but in the tail the wedge-shaped 

 half-moons are completed into bony rings and each vertebra 

 is represented by two of these rings. (A condition that will be 

 found of great interest in the consideration of the morphology 

 of the vertebras of certain of the amphibia.) The beginning of 

 the process of forming the bony vertebras by the growth of the 



