94 CLASS PISCES. 



carried to the surface. Sharks, Rays and flat-fish (with one 

 exception in each case) cease below 500 fathoms. Twelve 

 hundred fathoms is the limit for Holocephali. The eggs of some 

 deep-sea fishes ascend to and develop at the surface, but in 

 other cases the development undoubtedly takes place in the 

 abyss. 



Fishes are of great importance to our knowledge of the develop- 

 ment of animal life on the earth, owing to the frequent appearance 

 of their fossil remains in all geological periods. In the palaeozoic 

 formations very singular fish-forms, as the Cephalaspidae 

 (Cephalaspis, Coccosteus, Pterichthys), constitute the oldest 

 representatives of the Vertebrata. From the palaeozoic forma- 

 tions to the chalk we find almost exclusively cartilaginous fishes 

 and Ganoids, amongst which the forms with persistent notochord 

 and cartilaginous skull predominate. Ganoids, with a fully 

 developed bony skeleton, round scales and an externally homo- 

 cereal caudal fin, appear for the first time in the Jurassic rocks, 

 where we also find the first Teleosteans. From the chalk 

 onwards, in the more recent formations, the Teleosteans increase 

 in number and variety of forms the nearer we approach to 

 the fauna of the present time. 



The class Pisces is divided into the five sub-classes, 

 Marsipobranchii, Elasmobranchii, Ganoidei, Dipnoi, and 

 Teleostei. 



