THRACIA. 35 



on every part except the posterior side, which is covered with 

 numerous concentric rows of tubercles interspersed with fine 

 striae that appear to radiate from the tubercles : colour milk- 

 white : epidermis membranous, creamcolour : margins semi- 

 circular on the anterior side, moderately curved in front, with 

 a shght indentation or flexuosity towards the posterior side, 

 which is more rounded than truncated ; posterior dorsal margin 

 sloi)ing* and straight ; anterior dorsal margin slightly curved : 

 beaks projecting, with an abrupt excavation underneath, caused 

 by the compression or fracture of the hinge ; this part is de- 

 fined by a sharp but irregular ridge in each valve : ligament 

 exceedingly small (being only visible in fresh specimens), 

 placed close to the hinge on the posterior side; it is dark 

 horncolour : cartilage golden-yellow, contained in a triangular 

 and shallow cup, which is sohd, attached to the hinge-plate 

 by a ledge, and projects inwards horizontally and at a right 

 angle with the hinge ; from the lower part of this cup or car- 

 tilage-pit in each valve runs an obliciue and sharp ridge to the 

 posterior adductor muscle, and the shell is considerably thick- 

 ened in that part : hinge-line obtusely angular : hinge-plate 

 narrow and thin : ossicle falciform, clasping the hinge close to 

 the beak on the anterior side : inside chalky- white, except the 

 muscular scars and below the cartilage-pit, where the surface 

 is pohshed and nacreous ; it is furnished with a shght rib in 

 the line of fracture ; edges sharp : pallial and muscular scars 

 nearly marginal. L. 0*85. B. 1-3. 



Yar. curta. Shell more oval, or longer relatively to its 

 breadth. 



Habitat : Laiid^s End to Unst, from 4 to 60 f. ; and 

 at low water, spring tides, on the coasts of Kerry and 

 Galway. The variety is from Shetland and Cork Har- 

 bour. Fossil in the Coralline Crag (S. Wood) ; Chris- 

 tiania (Sars); Palermo (Philippi). The extra-Britisli 

 distribution comprises Iceland, the Faroe Isles, Scandi- 

 navia (3-30 f.), the north of France, Adriatic, Naples, 

 and Sicily. 



The course of striation, or the arrangement of the 

 microscopical granules, in this shell is the reverse of that 

 in Lyonsia Norvegicaj viz. transverse instead of longi- 



