SYSTEMATIC AND DESCRIPTIVE PART. 



The Class of Fishes is divided into four sub-classes : — 



I. Pal^ichthyes. — Heart with a contractile conus arte- 

 riosus ; intestine "with a spiral valve; optic nerves non-decus- 

 sating, or only partially decussating. 



II. Teleostei. — Heart with a non- contractile bulbus 

 arteriosus ; intestine without spiral valve ; optic nerves de- 

 cussating. Skeleton ossified, with completely separated 

 vertebrae. 



III. Cyclostomata. — Heart without bulbus arteriosus; 

 intestine simple. Skeleton cartilaginous and notochordal. 

 One nasal aperture only. No jaws ; mouth surrounded by a 

 circular lip. 



IV. Leptocaedii. — Heart replaced by pulsating sinuses; 

 intestine simple. Skeleton membrano-cartilaginous and noto- 

 chordal. No skull ; no brain. 



FIRST SUB-CLASS : PAL^^EICHTHYES. 



Heart with a contractile conus arteriosus; intestine with a 

 spiral valve -^ optic nerves non-decussating, or only partially 

 decussating ;^ skeleton cartilaginous or osseov^. 



This sub-class comprises the Sharks and Eays, and the 

 Ganoid fishes. Although based upon a singular concurrence 

 of most important characters, its members exhibit as great a 

 diversity of form, and as manifold modifications in the re- 



1 See p. 151, Kg. 67. ' See p. 128, Fig. 55. ^ gee p. loi. 



