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Show 1880.] FROM CENTRAL AFRICA. 351 three smaller muscular scars, irregular in form and position. Nacre beautifully iridescent, pinkish purple being the predominating hue. In young specimens a bluer tint prevails. Width 90 millims., length 43, diam. 15. The young shells take a very different form from the adult. They have quite an upcurved rostrated posterior end, such as is met with in certain species of Leda or Yoldia, e. g. L. patagonica, D'Orbigny, and Y. limatula, Say; but this form becomes modified as the creature enlarges its shell and the alation increases. Spatha alata of Lea, from Lake Nyassa, is closely allied to the present species, but differs in being more tumid, much more winged, having the anterior end more obtusely curved, and in lacking the elongate lamellar teeth. The presence of the latter might be regarded by some as a sufficient distinction to remove this species to another genus. The tout ensemble, however, of the shell is so very Spathoid that I prefer temporarily to locate it in that genus. 16. UNIO NILOTICUS, Cailliaud, Voy. a Meroe, vol. ii. pl. 61. f. 8. 9 ; Kiister, Conch. Cab. pl. 45. f. 5, narrow var.; Reeve's Con-chol. Icon. f. 374. U. parreyssii, Bush, Philippi's Abbild. vol. iii. pl. 5. f. 6. The specimens from Lake Tanganyika agree very well with this species. The form is subject to considerable variation, some specimens exhibiting a much more decided upper angulation at the posterior end, and having the curve of the lower margin different. The internal nacre also varies in colour, the general tone being whitish faintly tinged with a rosy blush. Two odd valves, however, are of a beautiful salmon tint, 17. UNIO TANGANYICENSIS. (Plate XXXI. figs. 9, 9 a.) Shell small, ovate, posteriorly acuminate, thickish, concentrically striated, and more or less corrugated in the anterior or lunular region, straw-colour, closely and finely rayed with bright green; anterior end regularly curved, forming an obtuse angle with the dorsal line ; ventral margin also much arcuated, feebly sinuated posteriorly, where the valves are rather acuminate ; posterior dorsal slope obliquely arcuate. Umbones slightly wrinkled at the apex and pearly. Valves with a narrow defined lunule anteriorly, and a shallow depression on the posterior side near the border. Hinge composed of comparatively fine roughened or striated teeth. Muscular scars irregularly rounded, anterior one deepest. Nacre generally of a purplish liver-colour, sometimes whitish and indistinctly rayed with pink, and rarely salmon-tinted. Length 17 millims., width 24, diam. 11. This is a very pretty little species, and remarkable on account of the vivid radiating green lines. The corrugation on the anterior end is somewhat granular in character. This wrinkling is only feebly expressed at the very apex of the umbones, and also slightly posterior to them. PROC. ZOOL. Soc-1880, No. XXIII. 23 |
