The smooth dog is easily identified among Gulf of Maine sharks by having two large spineless dorsal fins. The second only a little smaller than the first, combined with low, flat, pavement-like teeth. So different, indeed, are its teeth from the awl-like or blade-like teeth of all our other sharks that a glance at the mouth is enough to separate this species from the young of any larger Gulf of Maine shark. In form this little shark is slender, flattened below, with tapering but blunt snout. Its first dorsal fin originates nearly over the hind angle of the pectorals. The second dorsal fin is about twice as large as the anal, over which its stands. The tail is of typical "shark" shape, i. e. with upper lobe much longer than lower. The hind margin of the upper lobe of the caudal is deeply notched near the tip; the lower caudal lobe is very small.