With the exception of the nudibranchs, which are common all along the Atlantic shore, especially north of Cape Cod, there are very few opisthobranchs to be found in American waters. Their shells are not very abundantly found anywhere, though in Florida there are two or three species which an untrained collector might discover. Practically all the gasteropod or univalve shells that will be taken by the ordinarily expert collector along the shore are prosobranchs. This order includes the great majority of marine gasteropods, and is entitled to the distinction of claiming, in the great number of its genera and species, the most startling eccentricities of form and color to be found among the /Mollusca/, if not among all marine invertebrates. Only those species which are actually abundant on the Atlantic and Pacific shores of the United States, and which are likely to be found by the untrained collector, are selected from the long list of forms which belong to the three faunal regions involved. ORDER =OPISTHOBRANCHIATA= SUBORDER ~TECTIBRANCHIATA~ To avoid confusion, it is well to keep the systematic table in view: Class Orders Suborders =GASTEROPODA= =OPISTHOBRANCHIATA= =TECTIBRANCHIATA= =NUDIBRANCHIATA= =PROSOBRANCHIATA= =PULMONATA= [Illustration: PLATE LXV. Bulla occidentalis, enlarged. Bulla nebulosa. Lottia gigantea, inside view. Fissurella alternata.] As has already been remarked, shell-bearing opisthobranchs are not abundantly represented in number of species in North American waters. South of Cape Cod, in muddy bays and well-sheltered places, the little shell, /Haminea solitaria/, may, however, often be found in considerable numbers. GENUS ^Haminea^ ^H. solitaria.^ Like all external opisthobranch tests, the shell of this species is thin and fragile, and would appear to be of small importance to the creature it only partially protects. It is devoid of spire, is shining bluish-white, sometimes brownish in color, and is marked with revolving grooves, which, being cut across by irregular growth-lines, give its surface a faintly decussated appearance. The aperture is as long as the shell; the columella is incurved. This little species is especially abundant in shallow water about Vineyard Sound and in Peconic Bay. [Illustration: /Haminea solitaria./] GENUS ^Bulla^