“Tanganyika Shells”

Abstract

UNDER the above heading a paragraph appeared in NATURE, vol. xxv. p. 101, in which Mr. C. A. White, of Washington, states that certain species described by me in the Proceedings of the Zoological Society, 1881, pp. 558-560, from the great African Lake Tanganyika, “are without doubt, generically identical with the Pyrgulifera humerosa of Meek,” a fossil form from the Bear River Tertiary of North America. Mr. W. H. Dall, of the Smithsonian Institution, had previously, in a letter to me, dated October 24, expressed a similar opinion. I have been unable to procure for examination and comparison a specimen of the North American shell, and am consequently compelled to arrive at a conclusion from a study of Mr. Meek's figure and description in the report npon the “United States Geological Exploration of the Fortieth Parallel,” vol. iv., pp. 176-178, woodcut 6, and plate 17, figs. 19-19a. As a result I find it decidedly unadvisable at present to locate the two forms in question in the same genus. I admit that in regard to general outline and character of “sculpture” there is no distinction of any importance. However, when the aperture (which in univalve shells most frequently exhibits the main generic characters) is closely scrutinised, features present themselves which incline me, until actual comparison is possible, to hold these two types generically distinct. The outer lip of Pyrgulifera is said to be “subsinuous at the termination of the shoulder of the body volution above,” and the basal margin of the aperture is described as “faintly sinuous.” On the contrary, in Paramelania no trace of the latter character is present, and the upper extremity of the labrum where it joins the volution, instead of being “subsinuous,” is actually prominent. But another equally important distinction is the prolongation of the body-whorl below the aperture, together forming a more or less basal effusion. Independent of these actual differences, we must take into consideration certain probabilities and improbabilities. In the first place the difference in geographical position militates to some extent against the identity of these two forms. Then the vast lapse of ages surely must have evolved some differences in the animals as indicated by the dissimilar apertures, and asfain the operculum of Paramelania is very peculiar, and who shall say that this appendage was of a like nature in the Bear River shell. In conclusion, I should observe that the African form was considered of sub-generic rank by me, and not as a distinct genus, as stated by Mr. White.