>'UCUI.ID.«. 



33 



incliiiatis concentrice sculpta. Extrcmitas postica sensim at- 

 tenuata, liaud rostrata, supcnie angulata eininentiorque. Margo 

 dorsalis uterquc vix declivis, aiiticc coiivexiusculiis, postice recti- 

 usculus; ventralis ad extremitatcm posticam subito acclivis. Area 

 laiiceolata, acutu detinita, subla?vigata.— Elongated-obloiig, more 

 or less ventricose, only compressed on the convex-surfaced posterior 

 triangle, which (as well as, occasionally, the hinder umbonal region) 

 is devoid of the distant concentric sulDimbricating sulci which else- 

 where traverse the highly polished pale mud-yellow exterior. An- 

 terior side rather short," tilliiig about three-sevenths of the total 

 length, unsvmmetrieally rounded. Posterior side gradually at- 

 tenuated, not beaked, angulated and more prominent above, with 

 a slight dorsal notch or sinus : posterior edge convex, oblique. 

 Dorsal slopes trifling ; front one convex, or plano-convex ; hinder 

 one straight or subretuse. Ventral margin arching obliquely up- 

 wards in front, convex behind, bending abruptly up at its termi- 

 nation. Escutcheon lanceolate, carinately defined, more or less 

 smooth. Cartilage-pit sunken, triangular. Teeth numerous.— 

 In the variety Iqnda, the hinder side is more gradually attenuated 

 to a slightly more slanting tip, and the sulci are partially absent : 

 in the form serotbia the latter traverse the whole middle area, and 

 the lunular region is slightly more prominent. 



55. L. LEPiDULA, A. Adams, f. 6.— T. tenuis, L. tenella simil- 

 lima, sed inajor, magis expansa, et extremitate postica obtusiore 

 minusque ob'liqua ; cute pallide oiivaceo-cinerea iiiduta, plicae 

 umbonalis omnino expers. Margo dorsalis anticus ad extremita- 

 tem convexus. Arese dorsales incoiispicua3. Dentes utrinque 

 permulti.— Most closely resembling tenella, but larger, rather less 

 inequilateral, more spread and less ventricose in proportion. The 

 valves are thin, and gape at both extremities, the epidermis is of 

 a very pale olivaceous ash-colour ; the ventral margin, in the adult, 

 sweeps almost uninterruptedly, without central refusion, to the 

 hinder extremity, which is blunter and less oblique than in tenella. 

 There is not the least trace of an umbonal ridge. The front dorsal 

 edce is decidedly convex at the extremity. The beaks are pecu- 

 liiiriy small, and not all prominent. Dorsal areas inconspicuous. 

 Inside bluish-white, subiridescent : cartUage-pit small, triangular : 

 teeth very numerous on both sides, peculiarly narrow-based, and 

 elevated. 



56. L. TENKLLA, Hinds, f. 15.— T. insequilateralis, ovali-sub- 

 rhombea, antice inferne valde hians, postice superne rotundato- 

 angulata, eburnea, striolis arcti^ anticis longitudinalilius, mediis 



