Fishes of the Western North Atlantic 247 



Pelvics with nearly straight anterior and weakly concave posterior margins, narrowly 

 rounded apices and subacute tips, their origins considerably closer to origin of anal than to 

 origin of pectoral. Pectoral % as broad as long, or a little more, and a little larger in area 

 than 1st dorsal, with moderately convex outer and inner margins, nearly straight distal 

 margin, narrowly rounded apex and broadly rounded inner corner. 



Color. Adults in life are plain grayish olive, slaty-gray or brown above, without any 

 definite darker markings j yellowish or grayish white below with the margins of the fins 

 paler. In embryos, however, and in young specimens up to a length of 400 mm. or so, the 

 upper part of the first dorsal is more or less widely edged with dusky gray, the apex of the 

 second dorsal sooty-edged or tipped, its posterior margin white; the caudal with a sooty 

 blotch above at tip, white-edged below. The strength of these dark markings is variable, 

 however, and by the time a length of 600 to 700 mm. is reached they are only faintly dis- 

 cernible, or have wholly faded. 



Color Changes. This species is capable of changing shade — to a degree unusual among 

 sharks — by expansion or contraction of its melanophores, from dark gray above on a dark 

 background to a pale, translucent pearly tint when lying or swimming over a pale sand 

 bottom. It has been found by experiment that it requires about two days* for it to attain the 

 maximum paleness. 



Size. Smooth Dogfish range from about 340 mm. to about 390 mm. in length (aver- 

 age about 360 mm.) at birth. The majority of mature females with young are between 

 1,000 and 1,300 mm. (about 3 feet 3 inches to 4 feet 4 inches) long. The maximum length 

 is about five feet. 



Remarks. There is some evidence that in the tropical part of its range, where the 

 stocks of cants appear to be resident rather than migratory as they are in the north, local 

 populations may differ slightly from the typical form in their proportionate dimensions, 

 especially in the relative lengths of the labial folds, in the outlines of the fins and in the 

 sculpturing of the denticles. 



Developmental Stages. It has long been known that this species, like its close relative 

 M. mustelus of the eastern Atlantic, is truly viviparous, the embryo deriving its nourish- 

 ment from its mother by a highly organized yolk-sac placenta." The number of young in a 

 litter usually ranges between 10 and 20 (average about 16 in a large number of gravid 

 females recently examined at Woods Hole) , but litters as small as four have been reported. 



Habits. This is an inhabitant of the continental shelf and is not pelagic. During its 

 summer stay on the coast in the northeastern part of its range (see p. 249), it is most 

 commonly taken in comparatively shoal water of 10 fathoms or less, often coming into 

 enclosed bays and harbors, or even into fresh water on occasion.^" Large numbers of them 



8. For accounts of these experiments, some of which we have witnessed, see Parker and Porter (Biol. Bull. Wood's 

 Hole, 66, 1934: 30-37) and Parker, G. H. (Color Changes of Animals, Univ. Penn. Press, Sect. 2, 1936: 12-20). 



9. For account of this structure, see Fowler (Science, 50, 1909: 815) and Ranzi (Pubbl. Staz. zool. Napoli, /j, 

 1934: 387) for the European M. mustelus with list of earlier references. 



10. Definitely reported in fresh water in the North East River, Maryland, by Gunter (Amer. Midi. Nat., 28, 1942: 

 316). 



