1881.] LAKES TANGANYIKA AND NYASSA, ETC. 299 



53. Unio thomsoni. (Plate XXXIV. fig. 3G.) 



Unio thomsoni, Smith, Ann. & Mag. Nat. Hist. 5th series, vol. vi. 

 p. 430 (1880). 



Shell ovate, rather acuminated behind, concentrically striated, 

 granosely radiately distinctly corrugated in the dorsal region on hoth 

 sides of the umbones. The latter also prettily wrinkled in a zigzag 

 manner, small, rather acute, and placed well forward. Colour 

 yellowish or pinkish, faintly rayed with an obscure dull pinkish hue, 

 more or less hidden by a thin olivaceous epidermis. Valves not deep, 

 with a small anterior lunule, and a rounded narrow ridge on the 

 posterior slope, with a shallow depression on each side. Anterior 

 end regularly curve, posterior acuminate, ventral margin consider- 

 ably arcuate and indistinctly sinuate near the hinder end. Anterior 

 teeth small, irregularly bifid in one valve and trifid in the other, 

 roughened and striated ; posterior teeth slender, occupying about 

 half the extent of the slope from the beaks to the hinder extremity. 

 Interior of valves variable ; sometimes the nacre is bluish white 

 rayed with pinkish, and in other instances more uniformly purplish 

 brown. Length 1.5 millims., width 21, diam. 10. 



Hah. Lake Tanganyika (Thomson). 



In general aspect this species is similar to U. tanganyicensis, but 

 may be recognized bj' its smaller umbones, the radiating wrinkling 

 on hoth sides of them, and the greater amount of it upon them, 

 the greater curve of the ventral margin, the broader, more ovate 

 shape, and the absence of green radiating lines. 



Only three specimens of it were collected ; but they agree in the 

 above particulars, and have a peculiar tout ensemhle, which imme- 

 diately arrests notice if mixed with specimens of U. tanganyicensis. 

 The two depressions with a rounded ridge between, down tlie 

 hinder dorsal area, is another feature not occurring in that species. 



54. Unio horei. (Plate XXXIV. fig. 37.) 

 Unto horei, Smith, ibid. p. 429. 



Shell oblong, subrhomboidal, a trifle broader behind than in front, 

 inequilateral, rather thin, pearly, covered with a yellowish-olive 

 epidermis. Hinge-line straightish ; anterior end broadly curved ; 

 ventral margin only slightly arcuate ; hinder side oblique, very 

 feebly curved, forming an angle above with the hinder dorsal slope, 

 and a curve with the basal margin rather more sudden than that at 

 the opposite end of the valves. The latter are concentrically striated, 

 zigzagly corrugated towards the umbones, and radiately plicated on 

 each side of them ; and down the hinder dorsal area there are two 

 shallow depressions, which radiate from the umbones. Interior 

 white, pearly ; anterior and posterior teeth thin, lamellar ; scars shal- 

 low, anterior the deeper, subquadrate. Length 16| millims., width 

 25, diam. 9k. 



Ilab. Lake Tanganyika (Thomson). 



The single specimen of this interesting species, whose measure- 

 ments are above given, is in all probability but the young form of a 

 shell which attains a much larger size. However, it is quite distinct 



20* 



