On March 18, 1950, a single bleached skull of the porpoise described in this paper, was found by the senior author, on the beach above high tide line, along the north shore of Punta San Felipe, in the northern Gulf of California, Mexico. This skull is designated as the type specimen. Two other skulls of the species were available for study. These were obtained by James Warren and Paul Kellogg during December, 1951, at San Felipe Bay, Baja California Norte, Mexico, and are considered paratypes.
Seth Benson, of the Museum of Vertebrate Zoology at Berkeley, California, first noticed the striking differences that the type series shows, when compared with skull of Phocoena phocoena (auct. P. vomerina) in the MVZ collection. With this stimulus we have tried to obtain specimens of the entire animal, but have been unsuccessful thus far. We have, however, seen it alive, close at hand. On April 28, 1955, the senior author and James S. Long sighted what must have been Phocoena sinus, about 8 miles NE of Punta San Felipe, in the Gulf of California. This locality is a short distance out into the Gulf from the type locality. Three animals were seen. They were slightly less than 5 feet in length. Each animal was a uniform brown color dorsally and had a small triangular dorsal fin. Their blunt snouts were once observed. All were quite wary and were seen only briefly before they submerged. Our attention was first called to the animals by their sharp, loud breathing.