On Tylostoma , a proposed Genus of Gasteropodous Mollusks

Abstract

Among the organic remains of the beds belonging to the cretaceous system in Portugal are many casts of univalve shells, which have certain common characters entitling them to be classed together and distinguishing them from any described genus. Few of the specimens found retain any portion of the shell, and in no instance was I fortunate enough to find the shell perfectly preserved, so that the specific descriptions are necessarily imperfect; but there are among them materials to show the generic characters nearly complete. These shells are either globose or ovate, with a spire of moderate elevation, and resembling in form either the Globiconchae or certain Naticae and Phasianellae . The surface is nearly smooth. The mouth is nearly semilunate, with the lips united in a regular curve anteriorly and meeting above at a sharp angle. The outer lip is thickened internally by a callosity which reaches along its whole extent, and which, in some of the species, is slightly toothed : this internal callosity is repeated at regular intervals, which differ according to the species, the most frequent repetition being twice, and the most distant being once in a volution. The inner lip is expanded over the body-whorl, and almost conceals the columella, which is probably solid. At the periods of forming the internal callosity, the form of the mouth is temporarily modified by the outer lip being somewhat constricted, and by the aperture being lengthened at its upper extremity by a gradual rising of the upper edge of the