206 On new Mollusca from Aden. 



but possessing essential characters of its own. The shell is 

 equilateral, equivalve, almost round ; unibones slightly 

 oblique ; surface of a dull drab colour, here and there relieved 

 by bright brown spots or blotches, with about twenty-four 

 closely grained or scaly costaj ; these ribs are rather broad for 

 the size of the shell, and the interstices are narrow, very 

 rudely spirally scaled in a similar way to the ribs. The 

 hinge-teeth are whitish, about thirty in number ; the interior 

 is whitish, spotted and stained in places with rufous brown. 



Thracia adenensis, sp. n. 

 (PI. XII. fig. 1(3.) 



T. testa trapezoide, albida, papyracea, tenui, concentrice irregulariter 

 striata, parum nitida; umbonibusprominulis; valvadextrasinistrce 

 marginem ventialem suptrimpendeiite, majore ct coiivexiore, ab 

 umbonibus ad extremitutem marginis veutralis posticam unicari- 

 natfi, sinistra simili carina prsedita; pagina interna albo-lactea, 

 baud nitida, leevi ; cicatrice anteriore oblonga, postica squarrosa, 

 latiore, sinu palliali profundo. 



Long. 28, alt. 19, diam. 8 mm. 



A pure white papyraceous shell, extremely truncate poste- 

 riorly, with conspicuous unibones, that of the right valve 

 superimpending its fellow, just as the right ventral margin 

 overhangs that of the lelt. Tlie right valve is more convex, 

 but the left is not so flattened as in some Tliracice ; a con- 

 spicuous oblique keel in both valves extends from the umbones 

 to the lower posterior margin. 



EXPLANATION OF PLATE XII. 



Fig. 1. Ka^sa (Hebra) poll/chroma. 



Fig. 2. Mangilia (Glgp/wsfojua) femsfrata. 



Fig, o. Coralliophila urabica. 



Fig. 4. Aclis c.vccnta. 



Fig. 5. Fulijim SJia/)la7Hli. 



Fig. 6. Leiostraca co)istelhita. 



Fig. 7. Leptvthyra ycmenensis. 



Fig. 8. C'alyptrcea (Galenis) Edgariana. 



Fig. 9. JS'ovavuliiia .vyreces. 



Fig. 10. Tt'llina [TiHim-lIa) vianumissa 



Fig. 11. Tellina (Aiigulus) ycmenensis. 



Fig. 12. G'ari amianta. 



Fig. 13. 2'efricola Igra. 



Fig. 14. Seuiele S/ioj)/<i»di. 



Fig. 15. Pectunculus tigulicius. 



Fig. 16. Thracia adenensis. 



I 



