54 SUB-CLASSES AND ORDERS. 



confine ourselves to the first order, which is divisible into sub- 

 orders as follows : 



ACTINOPTERYGII : 



Gills lobed LOPHOBRANCHII. 

 Gills laminate. 

 Air bladder with duct. 



Skeleton notochordal ; body with dermal ossifica 

 tioris CHONDROSTEI. 



Skeleton osseous ; body without dermal ossifications 

 PHYSOSTOMI. 



Air bladder without duct. 



Skeleton incompletely ossified ; skin smooth, roughly 

 scaled or ossified PLECTOGNATHI. 



Some of the fin rays unarticulated ACANTHO- 



PTERYGII. 



None of the fin rays unarticulated ; ventrals, when 

 present, jugular or thoracic ANACANTHINI. 



The second sub-class, HOLOCEPHALI, now represented by only 

 three genera belonging to one family, is characterised by a single 

 external gill-opening with a rudimentary cartilaginous gill-cover 

 and four branchial clefts within the gill cavity, and by the jaws 

 being attached to the skull. 



The sub-class that includes the sharks and rays is of consider- 

 ably more importance. The body is more or less cylindrical or 

 depressed, the skeleton cartilaginous, the gills are attached to 

 the skin by their outer edges with from five to seven gill-openings, 

 there being no gill-cover, and the jaws being distinct from the 

 skull. The two orders are recognisable as follows : 



ELASMOBRANCHII : 

 Gill openings lateral SELACHOIDEI. 

 Gill openings inferior BATOIDEI. 



The first being the sharks, the second the skates and rays. 



Of course there are other distinctions in each case, but our 

 object being identification and not classification, we have mentioned 

 only those that are most noticeable. 



