On Neritoma , a fossil genus of Gasteropodous Mollusks allied to Nerita

Abstract

Among the fossil shells of the oolite hitherto referred to Nerita , there occurs a small group presenting characters of sufficient importance to justify their being separated, not only as a distinct section of the genus, but as forming a different generic type, probably belonging to the same family, for which I propose the name Neritoma . The peculiar character to which I allude is, in the outer lip (which in all the typical Neritae is entire) having two more or less deep sinuses, probably corresponding to a particular organization in the animal inhabitant; the form of the aperture and the columellar lip are also distinct from those of Nerita , and do not approximate it to any other described genus. The above-mentioned characters of this genus,—certainly allied to Nerita , although aberrant from it,—are interesting under two points of view: first, as connecting the true Nerites with Amphibola , Schum. ( Ampullacera , Quoy), also an aberrant form of Ampullaria and Natica ; and secondly, as adding another instance to certain genera of Mollusca, which with analogous forms present a similar character in having a greater or less sinus in the outer lip. In this latter respect, Neritoma bears the same relation to Nerita , as the other genera first mentioned in the following list do to their respective analogous forms. In consulting the above table of the two groups, it will be remarked that most of those forms furnished with a sinus belong to extinct genera: thus Acroculia , Murchisonia , Platychisma , are found in the palaeozoic series, Pleurotomaria