112 University of Washington Publications in Biology [Vol.2. 



3a. (See 3b, p. 114.) Gill openings 5 to 7, not covered by a large flap of skin 

 or bones (the operculum) ; skeleton cartilaginous. Fig. 2. 



4a. (See 4b, p. 113.) Gill openings wholly on the side of the body; body not 

 depressed into a disk but normal in form. Fig. 2. 



5a. Gill openings 6 or 7 on each side ; a single dorsal fin. 



Cow Sharks 3. Hexanchidae, p. 130 



5b. Gill openings 5 on each side ; 2 dorsal fins, the 2nd sometimes small. 



6a. Anal fin present ; dorsal fins not provided with spines. 



7a. First dorsal over or behind the pelvic fins. 



Cat Sharks 4. Scylliorhinidae, p. 131 



7b. First dorsal in advance of the pelvic fins. 



8a. Caudal fin not lunate ; side of caudal peduncle without keel ; 

 head and tail normal. Fig. 2. 



9a. Caudal fin forming less than l /z of the total length of the 

 fish; 1st dorsal nearly equal distance between space from 

 pectoral base to pelvic fin base; eye with a nictitating mem- 

 brane. 



10a. Spiracles obsolete ; distance from tip of snout to origin 

 of 1st dorsal is greater than the distance from origin of 

 1st dorsal to posterior margin of 2nd dorsal so that the 

 former distance extends a little beyond the 2nd dorsal fin. 

 Gray Sharks 5. Carchariidae, p. 131 



10b. Spiracles present; the distance from tip of snout to origin 

 of 1st dorsal a little less than distance from origin of 1st 

 dorsal to posterior margin of 2nd dorsal so that the for- 

 mer distance extends only to middle of 2nd dorsal fin. 

 Oil Shark. Soup-fin Shark.. 6. Galeorhinidae, p. 131 



9b. Caudal fin forming more than y 2 of the total length. 



Thresher Sharks 7. Alopiidae, p. 131 



8b. Caudal fin lunate, side of caudal peduncle with a well developed 

 keel. Fig. 3. 



11a. Dorsal fin just behind pectorals; gill slits about as long 

 as snout, not extending up the whole side of head ; gill 

 rakers short, not long and slender. 

 Mackerel Sharks 8. Lamnidae, p. 131 



lib. Dorsal fin about midway between pectorals and pelvics; 

 gill slits more than twice as long as snout, extending 

 up the whole side of head ; gill rakers long and slender, 

 resembling whale bone. 

 Basking Shark 9. Cetorhinidae, p. 131 



6b. Anal fin absent. 



12a. Dorsal fins each with a stout spine. 



Dogfish Sharks 10. Squalidae, p. 131 



