FOSSIL SHARKS 



77 



VIII. Digestive tube with a single bend, S, I, the 

 intestine provided with a spiral valve (p. 263), terminat- 

 ing, together with the ducts of the renal and reproduc- 

 tive organs, in a common cloaca, CL (p. 266). Liver, L, 

 spleen, and pancreas large ; mesenteries simple but greatly 

 fenestrated ; air bladder absent. 



IX. Heart with a contractile arterial cone, CA, con- 

 taining several rows of valves (p. 260) ; circulatory system 

 in general as described on p. 269. 



X. " Claspers " developed at the hinder margin of 

 the ventral fins as the intromittent organ of the male. 

 They are rudimentary in the female, CL' . Each clasper 

 is the trough-like hinder rim of the fin, which becomes 

 transformed into the compact, elongated, tube-like sperm 

 canal. Its tip is often studded with elongated shagreen 

 denticles whose recurved cusps retain it in copulo. 



Fossil Sharks 



Of all fishes, sharks certainly suggest most closely in 

 their general structures the metameral conditions of the 

 Cyclostome: it should also be noted that they possess the 

 greatest number of body segments, in some instances 

 over three hundred, known among vertebrates. Little is 

 known, however, of the primitive stem of the sharks, and 

 even the lines of descent of the different members of the 

 group can only be generally suggested. The development 

 of the recent forms has yielded few results of undoubted 

 value to the phylogenist : it would appear as if palaeon- 

 tology alone could solve the puzzles of their descent. 



The history of fossil sharks has as yet been but imper- 

 fectly outlined. The remains of the more ancient forms 

 have usually proven so imperfectly preserved that little 

 could be determined of their structural characters. Spines, 



