FISHES. 



325 



ORDER III. HOLOCEPHALI. 



A group of less than ten species of strange marine car- 

 tilaginous fishes in which the upper jaw is firmly united to 

 the cranium, the gills are covered by a flap of skin, like 

 an operculum, and a spiracle is lacking, compose this order. 



FIG. 124. Chimccra monstrosa. 



Mouth and nostrils are ventral, as in the sharks. The 

 name Chimcera, given to some forms, emphasizes their 

 strange appearance. Little is known of their habits. 



SUBCLASS II. GAXOIDEI. 



These are remnants of a group once very abundant on 

 the world's surface, but now showing less than fifty living 

 species in the whole world, and most of these in North 

 America. Some of them are much like Selachians, others 

 like Teleosts, and still others go off towards the Dipnoi. 

 The skeleton is bony or cartilaginous; the body may be 

 covered with ganoid or cycloid scales, or with bony plates, 

 or it may be naked; the tail either homo- or heterocercal; 

 the gills are covered with an operculum. The heart is 

 provided with an arterial cone, and the intestine has a 

 spiral valve. A swim-bladder occurs, and this has its 

 duct, which, in one form, empties into the ventral side of 

 the resophagus. With this confusing mixture of characters 



