The notched margin of the upper tail lobe distinguishes this shark at a glance from the spiny dogfish, with which it agrees in having a long pointed spine at the front edge of each dorsal fin. It differs further from the common dogfish in that its dorsal spines are deeply grooved along each side, whereas in the "dog" they are rounded; in the location of the pelvic fins, the rear axils of [page 52] which stand almost directly under the front origin of the second dorsal fin instead of some distance in front of the latter; in its small pectorals of rounded outline; in the shapes of its teeth, each of which has 3 or 5 sharp points; in its broad rounded snout; and in its very dark color. Like the spiny dogfish, it lacks an anal fin.
Adult specimens range from 2 to 31/2 feet in length, that is, about the same size as the spiny dogfish.
Uniform dark brown to black, below as well as above.
In West Greenland waters cephalopods, pelagic crustaceans, and medusae have been found in their stomachs, and females have been taken with embryos in February. Perhaps they are luminescent, for their skins bear minute deeply pigmented dots, suggesting the light organs of the brilliantly luminescent shark Isistius brasiliensis.
Northern North Atlantic; Faroe Bank, Faroe-Shetland Channel and Iceland in the east; West Greenland; Davis Strait; and outer slopes of the fishing banks in the west, southward to Georges Bank; chiefly deeper than 150 fathoms.
In the years when a long line fishery for halibut was carried on regularly, black dogfish were often caught along the slopes of the offshore Banks, from Grand to Browns and to the eastern part of Georges, if sets were made down to 200 fathoms or deeper. And while they dropped out of sight with the general abandonment of that fishery, no doubt they are as plentiful now as formerly, for we trawled about 100 of them, 6 to 241/2 inches long, off southwestern Nova Scotia, at 290 to 580 fathoms, on the Caryn of the Woods Hole Oceanographic Institution, in June 1949. How far they range to the west and south, at the appropriate depths, is not known.
 Its range has been said to extend to New York, but without supporting evidence; and report of a young one from the Gulf of Mexico (Goode and Bean, Smithsonian Contrib. Knowledge, vol. 30, 1895, p. 11), probably was based on some other shark.