A new west Mexican prosobranch mollusk parasitic on echinoids

Shells of two kinds of minute prosobranchs apparently belonging to the Stiliferidae were taken by the writer in 1949 from sand screenings at Cholla Cove near Puerto Pefiasco, Sonora, Mexico. These were duly assigned accession numbers and put to one side in the hope of obtaining better material before attempting intensive work upon them. A single poor specimen of one of these turned up in similar screenings in 1952 together with several shells of what may tum out to be a third member of the group, but it has remained for a friend, Mr. Harry R. Turver, collecting along the outer strand of the same playa early in 1954, again to find the species first referred to, this time in some numbers, alive, nestling amongst the aboral spines of the two large keyhole urchins, Encope grandis A. Ag.assiz (fig. 1) and E. californica Verrill, upon both of which it would consequently appear to be ectoparasitic. Both urchins