Dorsal to the tail +/- 36 ". Looking at some photos, dorsal to the tip of the tail is a little bit more than half the length of a dogfish. That would make your fish around 5-6 feet long. According to a NMFS publication, maximum size for a spiny dogfish is around 5.25 feet. So I guess it is possible that you saw a huge dogfish. But you said it didn't look like a dogfish - In what way?