4 BULLETIN 10 0, UNITED STATES NATIONAL MUSEUM 



Subclass ELASMOBRANCHII 



Membrane bones of head undeveloped, except sometimes rudimen- 

 tary opercle. Skeleton cartilaginous. Skull without sutures. Gills 

 not free, attached to skin by outer margin. No air bladder. Intes- 

 tine with spiral valve. Arterial bulb with 3 series of valves. Optic 

 nerve united by chiasma. Cerebral hemispheres united. Ova few, 

 large, impregnated, and sometimes developed internally. Embryo 

 with deciduous external gills. Skin naked or covered with minute 

 rough scales, sometimes with spines. Tail heterocercal. Ventral fins 

 abdominal. Male with large intromittent organs, or claspers, 

 attached to ventral fins. 



The true sharks and skates form an almost perfect gradation, 

 though the notidanoid sharks (Diplospondyli) are somewhat removed 

 from the former. Besides those included below, an extinct order of 

 sharks, the Ichthyotomi, with simple claspers, is known from early 

 fossils. The vernacular name "shark" is usually given as derived 

 from carcharus or carcharias^ with reference to the sharp teeth. 



ANALYSIS OF ORDERS 

 o^. Anal fin present. 



6\ Gill openings 6 or 7; dorsal fin single DIPLOSPONDYLI 



&*. Gill openings 5 ; dorsal fins double ASTEROSPONDYLI 



a*. No anal fin. 



&. Gill openings lateral. 



d\ Pectoral fins normal CYCLOSPONDYLI 



d^ Pectoral fins modified, large, expanded horizontally and extended for- 

 ward at base in front, which separated from neck by deep notch 



containing gill openings SQUATINAE 



&. Gill openings ventral RAJAE 



Order DIPLOSPONDYLI 



Vertebral column imperfectly segmented, each segment equivalent 

 to 2 vertebrae and bearing 2 neural arches. Gill openings 6 or 7. 

 Dorsal fin single. Anal present. 



The most primitive of existing sharks, with two families, recent 

 and extinct. 



ANALYSIS OF FAMILIES 



a\ Gill openings 6 or 7, their covers not crossing throat ; teeth compressed, blade- 

 like, notched on edges; mouth inferior Heptranchidae 



a^. Gill openings 6, their anterior covers crossing throat ; teeth with 3 long sharp 

 cusps ; mouth terminal Chlamydoselachidae 



Family HEPTRANCHIDAE 



Body moderately long. Head long, depressed. Snout protruding. 

 Eyes lateral, anterior, or submedian, without nictitating membrane. 



