OCR Text |
Show 1881.] LAKES TANGANYIKA AND NYAPSA, ETC. 285 length. Columella very arcuate, abruptly truncated at the lower extremitv. Probable length 41 millims., actual length of seven remaining whorls 36 ; diameter of last whorl 7§, of penultimate 7, of antepenultimate 65 ; aperture 7\ long, 4 broad. Hab. Near Lake Tanganyika {Thomson). The brevity of the aperture in proportion to the total length of the shell, its narrow elongate form, and the slow increase of the whorls constitute the chief characteristics of this species. Spiraxis bistorta of Pfeiffer has a considerable resemblance to it, hut has more convex whorls, a longer aperture, and a different columella. 20. SUBULINA SOLIDIUSCULA. (Plate XXXIII. fig. 16.) Subulina solidiuscula, Smith, Ann. & Mag. N. H . 1880, vi. p. 428. Shell elongate, subulate, rather solid, imperforate, white, covered with a thin olivaceous epidermis, obliquely striated by the lines of growth, which are crossed by shallow spiral striae, producing a somewhat granose or wrinkled surface. Lateral outlines rectilinear, slowly converging. Apex obtuse, truncated. Remaining whorls 8 in number, very slightly convex and slowly increasing, separated by a rather horizontal, simple, distinct suture. Aperture short, and the columella curved. Probable length of a perfect specimen 52 millims.; actual length of shell, consisting of eight volutions, 43 millims., diam. 11; aperture 9 long, 5| broad. Hab. Near Lake Tanganyika (Thomson). Of this interesting species only a single specimen was obtained. It is comparatively solid for a shell of this genus, and also remarkable on account of the very elongate form and the exceptionally slow increase of the volutions. The apex of the shell is obtusely truncated, but this may be the result of an accident in this instance, and not a constant specific character. The spiral or transverse striae are but feebly impressed, but crossing the fine lines of growth give them a crinkled appearance. LlMNOTROCHUS. Limnotrochus, Smith, ibid. p. 425. Shell trochoid, umbilicated, without an epidermis, spirally ridged. Body-whorl keeled round the middle. Aperture non-lirate within, with the outer lip oblique, the basal margin broadly sinuated, and the columella-edge somewhat reflexed and united to the labrum above by a callosity. Operculum horny, paucispiral, littorinoid. This remarkable form has all the appearance of a Trochus when viewed with the aperture from the eye. It is, however, more closely related to the Littorinidae, and exhibits the greatest affinity with the genus Echinella, from which, however, it may be distinguished by its operculum and the broad shallow sinuation in the lower margin of the aperture. 21. LIMNOTROCHUS THOMSONI. (Plate XXXIII. figs. 17-176.) Limnotrochus thomsoni, Smith, loc. cit. Shell moderately solid, narrowly umbilicated, trochiform, livid |