12 CONTRIBUTIONS TO NORTH AMERICAN ICHTHYOLOGY IV. 
Class III.-ELASMOBRAJ^CHII. 
{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 upi)er lobe i)roduced ; 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 
rndimeutary oj^ercle; skin naked or covered with minute imbricated 
scales or hard plates, sometimes sinnous ; 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 Chun®ras. 
{^ka(T/jLd7j a plate or blade; (ipdyyta, gills.) 
ANALYSIS OF OEDEKS OF ELASMOBRANCHS. 
• Gill-openings slit-like, 5 to 7 in number; jaws distinct from the skull. (Subclass 
Selachii.) 
t Gill-openings lateral Sqtjali, D. 
ft Gill-openings ventral Eaias, E. 
** Gill-openings single, leading to four brancbial clefts; jaws coa.lescent with the 
skull (subclass Holoccphali) Holocephali, F. 
Subclass SELACHII. 
{The SharJcs and Rays.) 
Elasmobranchiates with the gill-openings 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 Raice, the Bays, and Squall, 
the Sharks ; groups which are perhaps hardly worthy of ordinal value. 
{aildyoz, a shark, from aiXdyoq, cartilage.) 
Oedee D.-SQUALI. 
{The Sharlcs.) 
Gill-openings lateral, slit-like, five to seven in number; general form 
elongate, the body gradually passing into the tail. The typical sharks 
