22 



FISH GALLERY. 



the mother or within horny egg-shells (see specimens 7, 4, 5, 6 and 

 fig. 9). In the scheme of classification adopted in this Gallery 



Fig. 9. — Egg of a Dog-fish of the genus Scyllium. 

 (From Giinther, " Study of Fishes.") 



(see pp. 200-201) the Elasmobranchii are divided into four orders, 

 Proselachii, Acanthodides. Selachii, and Pleuracanthodes. 



PROSELACHII (Primeval Sharks). 



The Proselachii or Pleuropterygii are extinct Sharks, the most 

 primitive as well as the most ancient of the Elasmobranchii. The 

 only genus of which the general structure is at all well known is 

 Cladoselache (fig. 10), but detached teeth resembling those of 

 Cladoselache have long been known in the Carboniferous formations 

 under the name of Cladodus (see tooth of Cladodus striatus, 10). 

 The gill-slits are five or more in number • the upper and lower jaws 

 are approximately equal in size and are suspended from the 

 cranium by the hyomandibular cartilage. The dorsal and the 

 paired fins are supported by parallel fin-rays of calcified cartilage 

 which extend nearly to the margin of the fin. Dermal fin-rays 

 have been described in the posterior edge of the fin, but the 

 evidence of their presence is unsatisfactory (see pectoral fin, 8) . 

 The length of these Sharks varies between two and six feet. 



The remains of Cladoselache fyleri, found in the Cleveland 

 Shale (Upper Devonian or Lower Carboniferous) of Ohio, are 

 sufficient to show that the form of the body was rounded and 



