BIVALVIA. 261 



pubescens, I have followed their example, although the characters for specific separation 

 are not clearly defined, — the distinction appearing to rest solely upon one being more 

 transverse than the other. There is, I think, little doubt but the so-called two species 

 lived in the sea that deposited the Coralline Crag. 



Two or three fragments of what may be this species, or the young of the preceding 

 are in my cabinet, from the Red Crag of Sutton ; and a specimen belonging to this 

 transverse form is in the cabinet of Mr. Morris, from Uddevalla. 



Thracia detruncata, of my 'Catalogue' (fig. 1, e), is probably only a distorted 

 specimen of a young individual of either this or of the preceding species. 



The umbo is cleft by the ligament, which must have been visible when the valves 

 were closed. 



3. Thracia inflata, /. Sowerby. Tab. XXVI, fig. 6, a — e. 



Thracia convexa. 1 S. Wood. Catalogue, 1840. 



— inflata. /. Sow. Min. Conch., t. 631, figs. 2—4, 1845. 



— Conrari. 1 Couth. Bost. Journ. Nat. Hist., vol. ii, p. 153, pi. 4, fig. 2, 1839. 



Spec. Char. Testa obovatd, convexa, inflata, sublavigatd, tenui, fragili ; antice 

 rotundatd, postice subtruncatd et angulatd ; margine ventrali arcuato. 



Shell obovate, convex, tumid, nearly smooth, thin and fragile ; anterior side 

 rounded, posterior angulated and pointed ; ventral margin curved. 



Length, 3|- inches. Height, 2| inches. 



Locality. Cor. Crag, Sudbourn. 



This species appears to be restricted to the neighbourhood of Orford, where it is not 

 at all scarce, though the specimens are rarely in perfect condition : they are generally 

 more or less compressed, and the inflated character destroyed. Amongst my specimens 

 a considerable variation may be observed, some being much more elongated than 

 others. 



The right is the thinner and more inflated valve, and the one that is generally 

 fractured and compressed. Impressions by the adductors are unequal in size, and the 

 sinus in the mantle-mark is rounded and rather deep. A small sinus is visible at the 

 umbones, through which the ligament must have protruded ; but that part of the shell 

 being particularly thin, it is there generally destroyed or injured. The hinge is an 

 elongated callosity, on which was placed the cartilage, with a linear depression or 

 furrow on the outside of it for the ligamental portion, differing from that of T. pubescens, 

 in which this callus is of a triangular form and projects inwardly. The exterior is 

 smooth, with the exception of lines of growth and some rugosities, particularly upon 

 the siphonal side ; but it has not the shagreen-like sculpture which ornaments the 

 entire surface of T. pubescens. 



