5. SPIN ACID M. 15 



teeth numerous, in two or more series, the point so much turned aside 

 that the inner margin forms a cutting edge, which is entire ; spiracles 

 moderate ; no nictitating membrane ; gill-openings narrow ; fins all very 

 small, the veutrals nearly opposite the second dorsal ; skin uniformly 

 covered with minute tubercles. Species few, of the Northern Seas. 

 (Latin, somniosits, sleepy.) 



14. S. micvocephalaas (Bloch) Gill. Sleeper Shark; Nurse. 



Body robust, rapidly tapering behind; greatest depth about one- 

 fifth the length ; head somewhat less ; mouth moderate, upper jaw with 

 five rows of small sharp teeth, which are incurved and lancet-shaped ; 

 lower jaw with two rows of broad, quadrangular teeth, divided in their 

 centres by a perpendicular ridge and directed outwards, about 26 teeth 

 on each side; spiracles small; skin rough; fins small, the first dorsal 

 about as large as the ventrals and larger than the second dorsal ; pecto- 

 rals short; caudal short and bluutish. L. 8 to 20 feet (Storer). Cape 

 Cod to the Arctic Seas. 



(Squalus microcepJmlus Bloch, Schu. 135; Somniosus Irevipinna Le Sueur, Journ. Ac. 

 Nat. Sci. Phila. i, 222} Scymnus brevipinna Storer, Fish Mass. 235; Lcemargus borealis 

 Gunther, viii, 426.) 



FAMILY V. SPINACID^. 



(The Dog -fishes.) 



Body more or less elongate; head depressed; eyes lateral, without 

 nictitating membrane ; mouth inferior, rather large, arched ; teeth com- 

 pressed, variously formed ; nostrils inferior, lateral, near the front mar- 

 gin of the snout ; spiracles moderate ; gill-openings moderate, five on each 

 side, all in front of the pectoral fins ; dorsal fins two, eaeh armed with a 

 spine ; the first dorsal in front of the pectorals ; anal fin wanting; caudal 

 fin with, the lower lobe small or obsolete; ventral fins inserted pos- 

 teriorly. 



Genera six or more ; species about fifteen ; rather small sharks, chiefly 

 of the Atlantic. (Spinacidce, part, Gunther, viii, 417-425.) 



* Teeth eqnal in both jaws ; dorsal spines not concealed. 



a. Teeth very small, straight, pointed, each with one or two smaller cnsps on each 



* side CEXTROSCYLLIUM, 10. 



aa. Teeth moderate, simple, subquadrate, each with a nearly horizontal cutting 



edge, and a point directed outward SQUALUS, 11. 



* Teeth unequal, those in the upper jaw erect, triangular, those in the lower jaw 

 more or less oblique ; dorsal spines hidden beneath the skin. CENTROSCYMNUS, 12. 



