CONCRYLIA—DITHYRA. 23. 73 



Mactra testa trigond diaphand convexd subcequilaterali, lateribus trun- •-ineiea. 



catis, umbonibus prominulis incurvis. 

 Shell triangular transparent convex and nearly equilateral, truncate 



at the sides, with the beaks prominent and incurved. 

 Mactra cinerea. Montagu, Test. Brit. Suppl. p. 35. 



Pennant, Brit. Zool. iv. j). 196. 

 Found at Weymouth, by Mr. Bryer. 



Montagu is of decided opinion that this is a very distinct species, 

 presenting the same difference from the Mactra Stultorum as there 

 is between the Mactra solida and Mactra truncata, with resjiect 

 to the depth of the shell, prominence of the beaks, and flatness of 

 the truncated sides. 



The valves of this Mactra, he observes, are more concave 

 than of the Mactra Stultorum, more angulated, and rather 

 broader in proportion to their length ; the beak slightly turned to 

 one side, beneath which is a depression or subarcuation in its con- 

 tour, on the opposite side much compressed laterally : it is nearly 

 destitute of markings, there being only one or two obsolete jiale 

 rays, and the margin only covered with an epidermis, which is 

 brown inclining to ferruginous at one end : inside pale with a 

 tinge of blush. 



Specimens answering to this distinction are not unfrec[uently 

 cast on the Paington sands after an eastern gale. 



Mactra testa ovatd planiusculd incdquilaterali transversim striata, o•l;,^s:l. 



umbonibus incurvis. *• 



Shell oval flattish inequilateral and striate transversely, with the 



beaks incurved. 



l4 



