PROC. BIOL. SOC. WASH. 94(2), 1981, pp. 598-621 ANATOMY OF DIASTOMA MELANIOIDES (REEVE, 1849) WITH REMARKS ON THE SYSTEMATIC POSITION OF THE FAMILY DIASTOMATIDAE (PROSOBRANCHIA: GASTROPODA) Richard S. Houbrick Abstract. — Diastoma melanioides is the living survivor of a long lineage of snails in the Family Diastomatidae that occurred in the Tethys Sea during the Tertiary. Study of the anatomy shows that this species has open pallial gonoducts, and aphallic males, which establishes it in the superfamily Cer-ithiacea. A large ovipositor on the right side of the foot extends into the mesopodium. The radula is taenioglossate; the alimentary tract has paired salivary glands that run through the nerve ring and a stomach with style sac, gastric shield and reduced spiral caecum. The cephalic cavity is large and accommodates a small buccal mass and lengthy buccal and labial nerves. The nervous system is moderately condensed. Living animals occur subtidally in shallow water where they burrow in sandy bottoms and graze on algae and detritus. Development appears to be direct. Diastoma mela-nioides most closely resembles members of the Cerithiidae in anatomy and ecology. Anatomical and shell characters, and the fossil record indicate that Diastoma should be given familial status. The family is placed close to the Cerithiidae, the Potamididae, and the freshwater Melanopsidae. Introduction Diastoma melanioides (Reeve) is a relatively unknown cerithiacean prosobranch of moderate size which is restricted to a limited area of the coastline of the Great Australian Bight. It is the sole survivor of the Diasto-ma lineage, Family Diastomatidae. The taxonomic limits of this family have been poorly defined; consequently, a number of heterogenous groups have been referred to the Diastomatidae by numerous authors, and the family has been expanded well beyond the original concept. The living relict, Dias-toma melanioides, heretofore known only from its shell, is rare in most museum collections and unfigured in most publications. Recently, some well-preserved specimens collected at Esperance, Western Australia, were sent to me through the kindness of Mr. Alan Longbottom. This material has provided information about the operculum, radula, and anatomy of Diasto-ma melanioides which unequivocally establishes that it is a cerithiacean. The account that follows presents a description of the shell and internal
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