OCR Text |
Show 1881.] LAKES TANGANYIKA AND NYASSA, ETC. 289 horn-colour, banded with white beneath the suture. Whorls 12; first two or three convex, the rest nearly flat, slowly increasing, finely striated by the very flexuous lines of growth. Suture simple, hardly oblique. Aperture small, occupying nearly 5 of the total length. Labrum and columella as above described. Length 11^ millims., diam. 3; aperture 3 long, 2 broad. Hab. Lake Tanganyika (Thomson). Besides the lines of growth, some species show traces of spiral striation. 27. AMPULLARIA GRADATA, sp. nov. (Plate XXXIII. figs. 22, 22 a.) Shell globose, narrowly umbilicated, rather thin, moderately glossy, sculptured with oblique distinct lines of growth and minute spiral striae invisible to the naked eye, yellowish olive, with several bands and lines of a greenish tint. Whorls 6, depressed and flattened above, convex at the sides. Spire gradated, worn at the purplish- brown apex, equalling about one fourth of the total length. Aperture pyriform, whitish within at the upper part, and light brown elsewhere, with the bands and lines of a vivid dark brown colour: those on the upper part stop short at a little distance from the margin of the lip, leaving a narrow space of a sulphur colour; those lower down extend almost to the edge, where they are particularly bright. Lip thin, with scarcely any internal thickening. Columella below the umbilicus well curved, expanded, yellowish, connected with the upper extremity of the labrum by a very thin callosity. The following are the measurements of the three largest specimens :- Length. Greatest diameter. Length of aperture. Width. millim. millim. milliin. millim. 78 72 58 37 73 67 58 36 82 72 60 38 Hab. Lake Nyassa, and between it and the east coast {Thomson). The affinities of this species, if it be distinct, are rather with those forms found in Nilotic regions than with A. speciosa of Philippi from Zanzibar. The four species A. wernei, Philippi, A. kor-dofana, Parreyss, A. lucida, Parreyss, and A. ovata, Olivier, are very closely related ; and it is a matter of impossibility to define the limiting characters of any of them. The present species also may only be another form of the same shell. Like A. wernei and A. kor-dofana, the whorls are flattened at the top, so that the spire has a gradated appearance ; but from these it may be distinguished by its broader and shorter aperture and the considerably greater arcuation or incurvation of the columella; or the form may be described as more pyriform, being narrower above and broader below than in either of those two species. The banding within is of a deep tor-toiseshell brown, and only extends to the margin of the tip, along the base, and about halfway up the side. |