1881.] LAKES TANGANYIKA AND NYASSA, ETC. 285 



length. Columella very arcuate, abruptly truncated at the lower 

 extremity. Probable length 41 millims., actual length of seven 

 remaining whorls 36 ; diameter of last whorl 7|, of penultimate 7, 

 of antepenultimate 63 ; aperture 7| long, 4 broad. 



Hah. 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. Spiralis 

 historta of Pfeiffer has a considerable resemblance to it, but 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 oHvaceous epidermis, obliquely striated by the lines of 

 growth, which are crossed by shallow spiral striae, producing a some- 

 what granose or wrinkled surface. Lateral outhiies 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. 1 1 ; aperture 9 long, 5| broad. 



Hah. 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 remark- 

 able 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. 



LiMNOTROCHUS. 



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 cclumella-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 Littorinidse, 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, \7-17b.) 



Limnotrochus thomsoni, Smith, loc. cit. 



Shell moderately solid, narrowly umbilicated, trochiform, livid 



