The True Sharks 531 



ancestral sharks the vertebrae were without any ossification, a 

 simple notochord, possibly swollen at intervals. The dorsal 

 fin was single and long, a fold of skin with per haps a single 

 spine as an anterior support. The teeth must have been 

 modified dermal papillae, each probably with many cusps. 

 Probably seven gill-openings were developed, and the tail 

 was diphycercal, ending in a straight point. The finely 

 striated fin-spines not curved upward at tip, called Onchus from 

 the Upper Silurian of the Ludlow shales of England and else- 

 where, are placed by Hasse near his Polyspondylous sharks. 

 Such spines have been retained by the group of Chimeras, 

 supposed to be derived from the ancestors of Onchus, as well 

 as by the Heterodontidce and Squalida. 



Family Cochliodontidse. Another ancient family known from 

 teeth alone is that of Cochliodontidce. These teeth resemble 

 those of the Heterodontidce, but are more highly specialized. 

 The form of the body is un- 

 known, and the animals may 

 have been rays rather than 

 sharks. Eastman leaves 

 them near the Petalodontidce, 

 which group of supposed 

 rays shows a similar denti- 

 tion. The teeth are convex 

 in form, strongly arched, 



hollowed at base, and often Fl< f- 326. Lower jaw of Cochliodus contortus 



. , Agassiz. Carboniferous. (After Zittel.) 



marked by ridges or folds, 



being without sharp cusps. In each jaw is a strong posterior tooth 

 with smaller teeth about. The elaborate specialization of these 

 ancient teeth for crushing or grinding shells is very remark- 

 able. The species are chiefly confined to rocks of the Car- 

 boniferous ge. Among the principal genera are Helodus, 

 Psephodus, Sandalodus, Venustodus, Xystrodus, Deltodus, Pceci- 

 lodus, and Cochliodus. 



Concerning the teeth of various fossil sharks, Dr. Dean 

 observes: "Their general character appears to have been primi- 

 tive, but in structural details they were certainly specialized. 

 Thus their dentition had become adapted to a shellfish diet, 

 and they had evolved defensive spines at the fin margins, some- 



