THRACIA. 227 



that shell, was similar to, though smaller than, his own 

 pubescens. As, however, the last-named author has con- 

 fused our next species with the present one, and regarded 

 convexa as identical with distorta, we cannot look ujDon 

 him as a high authority upon the Thracias ; and the de- 

 scription of Pennant being so brief and inadequate, we 

 have, in accordance with the opinion of our best concho- 

 logists, preferred expunging altogether the name decUvis 

 to sanctioning the dangerous practice of hypothetically 

 determining an undefined object. 



This large Thracia has an oval-oblong figure, is rather 

 thin, inequivalve, subventricose, devoid of lustre, and of 

 a pale sand-colour ; the surface is distinctly shagreened, 

 and marked with coarse, irregular, concentric wrinkles ; 

 the ventral edge is almost straight, or even very slightly 

 retuse on the posterior side, and slightly convex on the 

 anterior. These distinctions are most apparent upon the 

 more ventricose valve, which overlaps the lesser one both 

 above and below. The sides of the adult are nearly equal, 

 the front one rounded, the hinder tapering and bluntly 

 biangulated at its termination ; the dorsal edge, which 

 anteriorly is very convex and moderately sloping, after 

 permitting by a slight concavity the insertion of the ash- 

 coloured and rather small sunken ligament, runs with but 

 little convexity and very moderate slope to the almost 

 straight posterior edge, the dorsal extremity of which is 

 rather the more projecting; there is a broad depressed 

 umbonal ridge, between which and the margin exists a 

 shallow excavation ; both these, however, are most readily 

 observable in the more convex valve, the umbo of which 

 is very prominent, and often worn away by the obliquity 

 of the incurved beak of the lesser valve ; the surface of 

 the hinder part of the shell is flattened near the dorsal 



