12 CONTRIBUTIONS TO NORTH AMERICAN ICHTHYOLOGY IV. 



Class III.-ELASMOBRANCHII. 



{The Selachians.) 



Skeleton cartilaginous ; skull without sutures. Body with median and 

 paired fins; the ventral fins abdominal; shoulder-girdle developed, lyri- 

 form ■ caudal fin heterocercal, the upper lobe produced ; gills attached 

 to the skin by the outer margin ; gill-openings several, or single leading 

 to several clefts ; membrane bones not developed, except sometimes a 

 rudimentary opercle; skin naked or covered with minute imbricated 

 scales or hard plates, sometimes spinous; no air-bladder; arterial bulb 

 with three series of valves; intestine with a spiral valve; optic nerves 

 united by a commissure, not decussating ; ovaries with the ova few and 

 large, impregnated, and sometimes developed, internally; embryo with 

 deciduous external gills; males with prehensile intromittent organs, 

 "claspers," attached to the ventral fins. Sharks, Skates, and Ohimseras. 

 (lAaff/j-o?, a plate or blade; (ipdy/Ja^ gills.) 



ANALYSIS OF ORDERS OF KLASMOBRANCHS. 



* Gill-opeuiugs slit-like, 5 to 7 iu uumber; jaws distinct from the skull. (Subclass 

 Selaclm. ) 



+ Gill-opeuings lateral Sqltali, D. 



ft Gill-openings ventral ..■ Rai^, E. 



•"Gill-openings single, leading to four brancliial clefts; jaws conlescent with the 

 skull (subclass Holoccpliali) Holocephali, F. 



Subclass SELACHII. 



[The Sharls and Bays.) 



Elasmobriinchiates with the gill-openiugs slit-like, five (rarely six or 

 seven) in number ; jaws distinct from the skull ; no opercular nor pelvic 

 bones ; derivative radii sessile on the sides of the basal bones of the limbs, 

 rarely entering the articulation. 



As here understood, this subclass is equivalent to the Plaglostomata of 

 authors, and includes the orders or suborders Eaia', the Eays, and Squali, 

 the Sharks ; groups which are perhaps hardly worthy of ordinal value. 

 {ffiXd/o^, a shark, from aiXaxo^^ cartilage.) 



Order D.-SQUALI 



{The Sharhs.) 



Gill-openings lateral, slit-like, five to seven in number; general form 

 elongate, the body gradually passing into the tail. The tyx)ical sharks 



