neio Molluscafrom Aden. 205 



umbones are shining, pure white ; dorsal margin rather 

 sloping, anterior rounded, ventral slightly convex ; valves 

 both equally tumid. Within, the surface is white, the im- 

 pression of the posterior adductor muscle is large and round, 

 the pallial line deeply sinuous posteriorly. 



Far more conspicuously ribbed and acutely keeled than 

 P. Hempriclni, Issel, which is the prevailing form at Aden 

 {cf. A. Issel, Mai. del Mar Rosso, 1869, pp. 61, Q2, pi. i. 

 fig. 6). This latter species, indeed, is not far removed from, 

 if not absolutely identical with, the common Mediterranean 

 form, P. lithophaga, L. 



Semele Skoplandi, sp. n. (PI. XII. fig. 14.) 



S. testa nitida, albida, tenui, breviter ovata, lunula angusta, oblonga ; 

 umbonibus parvis, laevibus ; margine dorsali utrinque declivi, 

 postice subtruncato, antice orato; margine ventrali rotundo-con- 

 vexo, superficialiter fere laevi, conceutrice bi- vel tri-zonulato, et 

 leniter decussate vel sculpto ; intus albo-lactea, nitida, sinu palliali 

 conspicuo, extus medium extenso. 



Alt. 26, lat. 28, diam. 13 mm. 



A pure white delicate species with inconspicuous umbones 

 and oblong narrow lunule, the dorsal margin sloping rapidly 

 on both sides, posteriorly subtruncate and ovate anteriorly, 

 the ventral margin rounded, convex. Within, the shell is 

 white, the pallial line possessing a conspicuous sinus ex- 

 tending to the middle of the shell. The outer surface is 

 smoothish, rather shining, white, concentrically twice or thrice 

 rudely banded ; under a lens the whole surface is thinly 

 decussate with scratched lines. 



Superficially only allied to S. cordifonni's, chinensis, &c., 

 the valves being smoother than in any other Eastern species. 



Pectunculus tegidicius*, sp. n. (PI. XII. fig. 15.) 



P. testa aequilaterali, fere rotunda, incrassata, convexiuscula, sordide 

 ciuerea, hie illic macuHs rufis interspersis ; umbonibus obliquis ; 

 radiatim fortiter costata, costis arctis, quatuor et viginti, imbri- 

 catis et transversim crassi-squamatis, interstitiis angustis, squamis 

 rudibus similiter iustructis ; cardine dentibus trigiuta in valva 

 utraque praedito, interiore albesceute, rufo-sparso. 



Alt. 18, lat. 20, diam. 13 mm. 



A very interesting form, though, perhaps, not quite mature, 

 * TeyuUcius, of, or pertaining to, a tiled roof. 



