FAMILY SQUALID^. 355 



GENUS MUSTELUS. Cumer. 



Teeth blunt, forming a closely compacted pavement in each jaw ; with temporal orifices. 

 First dorsal in advance of the ventrals. Lower lobe of the caudal short. No spines. 



THE AMERICAN HOUNI>FISH. 



>. 



MuSTELUS CANIS. 



PLATE LXIV.— FIG. 209. 

 Dog-fish, Squalus canis. MiTCHiLL, Trans. Lit. and Phil, Soc. Vol. 1, p. 486. 



Characteristics. Unspotted ; dull ash grey. Upper edges of dorsals and anal bordered with 

 black. Length two to four feet. 



Description. Body slender, cylindrical, tapering, elongated. Head flattened above ; snout 

 thin, obtusely pointed. Surface of the body like shagreen when rubbed forwards, and appears 

 under the lens to be composed of acute spinous plates. Lateral line scarcely obvious, nearer 

 the back than to the abdomen. On the back, at a point above the origin of the pectoral fins, 

 is a slightly elevated ridge, only interrupted by the fins, and extending to the origin of 

 the caudal fin. The two posterior branchial apertures over the base of the pectorals ; 

 the fourth longest, the fifth shortest, and subequal with the first. Temporal orifices circular, 

 about ■ 2 in diameter ; they lie in a fossa, and are distant " 3 from the posterior angle of 

 the eyes. Eyes oblong, lateral, 0.45 above the angle of the jaws ; vertical diameter 0"4, 

 transverse diameter 0-7. Nostrils large, semilunar, on the underside of the snout, and 

 midway between the angles of the mouth and the tip of the snout. Anterior margin with a 

 large and loose triangular process, and a small one external to it ; attached to the externa? 

 side, anil to the lower part of the anterior margin, is a second stout process, with a small 

 membranous appendix ; another process, longer than the preceding, is attached to the inside 

 of the psterior niargin. When these several valvular processes are put aside, we are enabled 

 to observe the curiously pectinated nasal apparatus. A series of minute punctures originate 

 a short distance before the branchial openings, and terminate 0' 5 behind the nostrils. Similar 

 punctures (the openings of mucous ducts) are observed to be irregularly distributed on the 

 under side of the head, anterior to the mouth. The mouth crescent-shaped when opened, 

 triangular when closed. Upper lip elongated, free ; lower lip convoluted. Teeth on the jaws, 

 smooth, flattened, rhomboidal, distributed in about ten rows ; the posterior edge of each tooth 

 slightiy elevated, so as to produce a distinct roughness when the finger is drawn forward. 

 Tongue moderately free, thin and rough ; its form subtriangular, obtusely truncated at the tips. 



Both dorsal fins nearly of the same shape ; the first 3 " 5 high, its base 2 " 5, quadrilateral, 

 excavated behind, with an elongated point behind ; its origin is eight inches from the end of 

 the snout, and slightly behind the base of the pectoral fins. Second dorsal smaller, and 

 arising seventeen and a half inches from the point of the snout; it is 2-7 high, with a base 



