THRACIA. 41 



It is the T. declivis of Macgillivrav, T. ventricosa of 

 Philippic and (apparently) the T. ScheejimaJceri of 

 Dunker. 



B. Posterior side usually larger. 

 5. T. distor ta*, Montagu. 



My a disforta, Mont. Test. Brit. p. 42, t. i. f. 1. T. distorta, F. & H. i. 

 p. 231, pi. xvii. f. 1, 2, 3, 8, and (animal) pi. H. f. 5. 



Body roundish-oval, white : tubes rather short, and some- 

 what more united towards their bases than in the other British 

 Thracice ; the branchial or lower tube is often extended more 

 than half an inch, while the other remains quiescent ; pre- 

 viously to the former being withdrawn, it is always globularly 

 inflated at its extremity, which inflation increases until it ex- 

 tends near the margin of the shell, and the tube then suddenly 

 collapses ; during the inflation the terminal cirri disappear, 

 and they only become visible when the tube is at rest : gills 

 large and brown : palps nearly equal and pectinated : foot 

 short and linguiform. 



Shell varying in shape from round to oval, more or less 

 distorted and often sinuous, generally convex but sometimes 

 flattened, more solid in proportion to its size than the other 

 species, opaque and lustreless : sculpture, minute and crowded 

 tubercles or granulations of equal size, arranged' in concentric 

 although irregular rows ; marks of growth distinct : colour 

 milk-white, with occasionally a yellowish tinge : epidermis 

 membranous, abraded in front and only to be seen at the 

 edges, dingy brown : margins rounded on the anterior side 

 and in front, somewhat truncated or wedge-shaped on the 

 posterior side, which is in most instances (but not invariably) 

 larger or more elongated than the other side, and obscurely 

 angulated ; dorsal margins obtuse-angled : beaks sharp and 

 entire, slightly inclined to the posterior side ; umbones ra- 

 ther prominent : ligament short, of various shades of colour 

 from yellowish to dark brown, separating the valves by an 

 oval gap : cartilage strong, yellowish -brown or horncolour, 

 contained in a thick triangular receptacle, which is set ob- 

 liquely and projects considerably within each valve ; fulcral 



* Distorted. 



