296 MALACOZOA. THOPIOPODA. LAMELLIBRANCHIATA. 



and more gaping behind. Umbones small, rather pro^ 

 minent. In each valve a horizontal, oblique, concave 

 tooth, receiving an internal hgament, of which the pos- 

 terior side gives attachment to a semiannular calcareous 

 spiculum ; the hgament appearing externally. Muscular 

 impressions small, submarginal. 



This genus is very nearly allied to Amphidesma, as 

 well as to Anatina. 



1. Thrdcia dedivis. Sloping Thracla, 



Shell oval, convex, very thin, brittle, subdiaphanous, with 

 the anterior end larger and rounded, the posterior narrowed, 

 compressed, roundly truncate, somewhat gaping ; umbones 

 prominent, recurved, pointed, andmeetmg; frontal slope neai-ly 

 straight, dorsal mvich deciinate, straightish ; an obtuse ridge 

 from the umbo to the lower angle of the truncation ; the sur- 

 face concentrically rugoso-striate, v>'ith a very thin greyish- 

 yellow epidermis ; the cardinal lamina very narrow, adnate, 

 anteriorly rovmded, a deep sinus under the umbo, arid before 

 it a narrow, posteriorly rounded, concave, adnate lamina ; the 

 inner surface dull, white, with a naiTow pearly space from the 

 mnbo backwards. Length about two inches and a-half, height 

 an inch and a-half. 



A fractured single valve found on the 27 th of September, 

 1842, by Mr. Leslie at Footdee, brought from deep water, off 

 Aberdeen, by the lines. 



Mya declivis. Penn. Brit. Zool. iv. 79. — Mya declivis. Donov. 

 Brit. Sh. iii. PI. 82.— Anatina convexa. Tm-t.'Brit. BIt. 44. PI. 4. 

 f. 1, 2.— Anatina my aUs. Lamk. Syst. v. 464; Eel. 2. vi. 80.— 

 Amphidesma conTexum. Flem. Brit. Anim.431. 



This species has been singularly misunderstood by authors. 

 The above references however are all certain. Pennant's short 

 description is perfectly correct: "Mya with a brittle, half- 

 transparent shell, with a hinge slightly prominent ; less gaping 

 than the truncata ; near the open end sloping downwards. ' 

 He adds, "Frequent about the Hebrides; the fish eaten by 

 the gentry ;" but these statements are incorrect. 



2. Thrdcia puhescens. Roiighish Thracla, 



Shell ovato-oblong, somewhat compressed, very thin, brittle, 

 pellucid, with the anterior end larger and rounded, the pos- 

 terior truncate ; concentrically faintly striate ; the surface 



