POROMYA. 45 



The structure of the shell is very remarkable, althoug^h 

 not differing much from that of T/iracia. The external 

 layer consists of crowded oblong cells having their ends 

 outward, and the inner layer is nacreous ; the cellular 

 part is easily rubbed off. The mantle is said to be open 

 in front, an unusual character in this group. Further 

 ])articulars of the animal are desirable. 



This genus is the Embla of Loven, and (according to 

 Chenu) the Eucharis of Eecluz. 



PoROMYA granula'ta*, Nyst and Westendorp. 



Corbula granulato, Nyst & West. Coq. Foss. d'Anvers, p. 6, pi. 3. f. 3. P. 

 granulata, F. & H. i. p. 204, pi. ix. f. 4-G, and (animal) pi. W. f. 2. 



Body crcamcolour : mantle open in front : tubes encircled at 

 their bases by a fringe of 18 or 20 tentacular filaments, which 

 expand like the petals of a flower, and are sometimes folded 

 back on the posterior side of the shell : foot very transparent. 



Shell somewhat quadrangular or rhomboidal (the right 

 valve larger than the left and slightly overlapping it), mode- 

 rately convex, fragile ; externally it is opaque and of a dark 

 hue, but when the superficial or granular coating is removed, 

 it is semitransparent and glossy : scidptiire, very minute and 

 close-set tubercles of nearly equal size, arranged in longitu- 

 dinal rows, and occasional but slight marks of growth : colour 

 dusky outside, and whitish imder the surface-layer : epidermis 

 dark brown, visible only at the edges, and especially at the 

 back (where it forms a kind of elongated ligament on both 

 sides of the beak) : margins rounded on the anterior side, 

 shghtly curved in front, indented near the posterior side, which 

 is obliquely truncated and has a distinct ridge extending from 

 the beak to the posterior angle, with a broad fold on either 

 side of it ; posterior dorsal margin longer and straighter than 

 the other : heahs blunt and calyciform ; umbones prominent : 

 cartilage yellowish -brown, set rather obliquely in an obtuselj^ 

 angular receptacle, which does not project far within : hinge- 

 line gradually curved : hinge-plate thickened on both sides of 

 the beak : teeth, in the right valve an erect, blunt and tuber- 



* Gninulated. 



