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Show 1881.] LAKES TANGANYIKA AND NYASSA, ETC. 299 53. UNIO THOMSONI. (Plate XXXIV. fig. 36.) 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 both 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 considerably 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 15 millims., width 21, diam. 10. Hab. Lake Tanganyika {Thomson). In general aspect this species is similar to U. tanganyicensis, but may be recognized by its smaller umbones, the radiating wrinkling on both 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 ensemble, which immediately arrests notice if mixed with specimens of U. tanganyicensis. The two depressions with a rounded ridge between, down the hinder dorsal area, is another feature not occurring in that species. 54. UNIO HOREI. (Plate XXXIV. fig. 37.) Unio 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 shallow, anterior the deeper, subquadrate. Length 16^ millims., width 25, diam. 9|. Hab. Lake Tanganyika (Thomson). The single specimen of this interesting species, whose measurements 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* |