229 



Thracia concentrica, n. sp. PI, xxviii., figs. 12, l:'*, 14. 



Shell, rhomboid-oval ; anterior-dorsal border straight, 

 scarcely convex; posterior-dorsal border rather more concave; 

 ventral-border uniformly slightly convex, joining the ant<jrior 

 border with a sharp curve ; posterior end nearly vertically 

 truncate, scarcely rounded at the superior angle, more at 

 the inferior. A rounded umbonal ridge curves to the pos- 

 terior inferior angle. Numerous valid, round, rude, incom- 

 plete, ill-curved concentric ribs frequently overlap in the 

 median area. Fine crowded, crooked, radial incisions cross 

 the ribs and spaces, and interrupt more crowded concentric 

 incisions, so as to produce a microscopic shagreen pattern. 

 There is a depressed lanceolate dorsal area behind the umbo, 

 at the margin of which the ribs cease, and which is finely 

 antero-posteriorly striate. In the right valve is a very nar- 

 row furrow within the dorsal borders for the edge of the 

 left valve, and its inner margin behind the umbo projects as 

 a laminar tooth, soon vanishing posteriorly. The inner sur- 

 face is glistening, and has faint radial substriations. The 

 pallial sinus is of moderate depth and round. 



Dimensions. — Umbo-ventral, 10'8 mm. ; antero-posterior, 

 14'1 : sectional, 4*9. 



It has not been dredged alive or with united valves, and 

 one cannot be certain about its left valve. 



It closely resembles T. myodoroides, of Smith, but is 

 more ventricose, has a concave post-dorsal border, and is much 

 more validly concentrically ribbed. 



Habitat. — 23 fathoms. Backstairs Passage. 



Thracia lincolnensis, n. sp. PI. xxviii., figs. 19, 20, 21. 



Shell, thin, white, elongate-oval ; anterior-dorsal border 

 nearly straight, scarcely convex ; posterior cancave ; ventral 

 border almost straight in the posterior half, arcuately ascend- 

 ing in the anterior. Anterior end attenuatelj rounded, pos- 

 terior end vertically, straightly truncate ; superior and in- 

 ferior posterior angles rounded. Right valve slightly more 

 convex than the left, which is included all round except at 

 the posterior end. Quite obsolete, flat, irregular concentric 

 ribs, with very faint miscoscopic concentric striae. The right 

 valve has a groove inside its dorsal margin, except at the 

 umbo, to receive the left valve, and the inner edge of the 

 groove projects to form lamellar teeth, most marked just in 

 front of and behind the subumbonal notch. The pallial sinus 

 is wide and round, and reaches nearly to the centre of the 

 shell. The umbos are acute, incurved, and apposed. 



Dimensions. — Antero-posterior, 7*3 mm.; umbo-ventral, 

 5*1 mm. : sectional, 3 mm. 



