IHBO.] FROM CENTRAL AFRICA. 351 



three smaller muscular scars, irregular ia 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. Thev 

 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 crea- 

 ture 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 re- 

 garded 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. 



IG. Unio niloticus, Cailliaud, Voy. a Meroe, vol. ii. pi. 61. 

 f. 8, 9 ; Kiister, Conch. Cab. pi. 45. f. 5, narrow var. ; Reeve's Con- 

 chol. Icon. f. 374. 



U. parreyssii. Bush, Philippi's Abbild. vol. iii. pi. 5. f. 6. 



The specimens from Lake Tanganyika agree very well with this 

 species. The form is subject to considerable variation, some spe- 

 cimens 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, feeblv sinuated 

 posteriorly, where the valves are rather acuminate ; posterior dorsal 

 slope obhquely arcuate. Umbones shghtly 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. Mus- 

 cular scars irregularly rounded, anterior one deepest. Nacre generally 

 of a purplish liver-colour, sometimes whitish and indistinctly raved 

 with pink, and rarely salmon-tinted. Length 1 7 millims., width 24, 

 diam. 1 1. 



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 wrinkUng is only feebly 

 expressed at the very apex of the umbones, and also slightly pos- 

 terior to them, 



Proc. Zool. Soc— 1880, No. XXIII. 23 



