CORNIFEROUS PERIOD. 



277 



these have been thence called heterocercal fishes, while those 

 having a tail of the ordinary form, as in fig. 473, are said to be 



homocercal.* 



* Fishes. — A systematic review of the class of Fishes will make the subject 

 more intelligible. The relations of Fishes to other Vertebrates are mentioned 

 on p. 152. Fishes (excluding a few abnormal kinds of the group Dermopteri) 

 are divided into three orders : — 



I. Selachians (or Placoids). — Include the Sharks (figs. 462, 464) and Rays, 

 having (1) the skeleton cartilaginous ; (2) the body covered usually with a 

 harsh skin; (3) the gills attached by both margins, and the gill-openings 

 without opercula (g, fig. 462) ; (4) the optic nerves not decussating. The term 



462-472. 



Selachians.— Fig. 462, Spinax Blainvillii (X Vs) 5 463, Spine of anterior dorsal fin, natural 

 size; 464, Cestracion Philippi (X Ys)\ 465, Tooth of Lamna elegans ; 466, Tooth of Car- 

 charodon augustidens ; 467, id. Notidanus primigenius; 468, id. Hybodus minor ; 469, id. 

 Hyb. plicatilis; 470, Mouth of Cestracion, showing pavement-teeth of lower jaw; 471, 

 Tooth of Acrodus minimus ; 472, id. Acrodus nobilis. 



Selachian — proposed by Cuvier, and now used by Agassiz — is from the Greek 

 szkaxos, cartilage, while Placoid is from n\al, a plate. The rough skin is often 



