. Bulletin - United States National Museum. Science. 7':astern rROViNci:—ixtkriou region species. 265 Fig. Fig. 283. iiaiiie of II. iStrebelij Pfr. (see Fischer and Crosse, Moll. Mex. et Gnat.). ]\Ir. Morse has ^iven the following description of the interuallaminsB which characterize this species: " The shell has been described as having one revolving tooth within the aperture, and sometimes a second one, ter- minating farther within the aperture. I have always found this second one constant, and also a ihird one, but slightly raised between these two. At the base of the shell and f
. Bulletin - United States National Museum. Science. 7':astern rROViNci:—ixtkriou region species. 265 Fig. Fig. 283. iiaiiie of II. iStrebelij Pfr. (see Fischer and Crosse, Moll. Mex. et Gnat.). ]\Ir. Morse has ^iven the following description of the interuallaminsB which characterize this species: " The shell has been described as having one revolving tooth within the aperture, and sometimes a second one, ter- minating farther within the aperture. I have always found this second one constant, and also a ihird one, but slightly raised between these two. At the base of the shell and fiir Avithin the aperture are two more revolving ribs, run- ning about a third of one volution. These are plaiidy visible through the substance of S-^-^yrintMca, the shell. A heavy columellar tooth or rib extends from a slight dis- tance within the aperture, nearly one volution back. This columellar tooth thickens the substance of the shell in the umbilical region, and causes a dis- tinct fold without the shell. A most singular feature is revealed in the structure of the parietal lamina',. With an ordinary magnifying ])ower small swellings are seen at close intervals along these laminse, which, when magnified four hundred diameters, are seen to be surmounted with from five to ten sharj) spines, pointing toward the aperture. Tliese swellings appear to coincide in number and position with the raised ribs without the shell, though they are not formed at the same time, for as these laminse approach the aperture they become attenuated and disappear. The surface upon which these laminae rest is granulated, and not smooth, as is generally the case with the interior of shells. It is difficult to imagine the use of these spiny projections, unless they may act in some way as points of resistance to the animal for the sup- port of a very heavy ; Jaw: see p. 203. Lingual membrane with 78 rows of teeth each; centrals tri- cuspid, central cusp very long; latera
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