BIVALVIA. 65 



The recent shell is said to be as much as a quarter of an inch in diameter : 

 the largest of my fossil specimens, does not exceed the sixth of an inch in its 

 extreme dimensions, measured diagonally from the umbo to the posterior part of its 

 ventral margin. There is, nevertheless, but little doubt of their identity, as the 

 only difference is that of size, the Crag specimens corresponding in all other charac- 

 ters with the recent shell. It is very much inflated : the two valves when united, 

 having a greater diameter than is given when measuring from the dorsal to the 

 ventral margin ; the umbo is terminal, projecting a little beyond the anterior of the 

 shell, and is slightly curved ; it is somewhat rhomboidal in its contour, with a little 

 obliquity towards the posterior side, and a slight indentation in the ventral margin, 

 the posterior side forming an obtuse angle with the edge of the shell that contains the 

 ligament ; the exterior is ornamented with large prominent striae, or rather small ribs 

 which bifurcate, and are more numerous in the old than in the young shell, they 

 are crossed and made somewhat rough by prominent and distinct lines of growth, 

 at rather irregular distances, sometimes giving a decussated appearance to the shell ; 

 the prominent costulated striae project beyond the margin, and give a deeply 

 crenulated edge all round; the ribs are sometimes visible in the interior, although 

 the specimens are often so thick as not to allow them to be seen on the inside ; and 

 in that case, the impressions formed by the muscles are deeply indented, that by the 

 anterior adductor is comparatively very large. The ligament seems to have been 

 a strong one, as a deep linear depression is formed within the dorsal margin. This 

 shell has been dredged in the living state, in 20 fathoms water, off Penzance. 



PECTUNCULUS,* Lamarck, 1791. 



PECTUNCULUS POLYLEPTOGINGLXMUS (sp.) List., 1687. 



MACTRA ? Browne, 1756. 



ARC A (spec.). Linn., 1767. 



GLYCIMERIS. Da Costa, 1778. Humph., 1797. 



AXIN.EA AXIN^EODERMA. Poll, 1795. 



TUCETA. Bolton, 1798, sec. Herrm. , 



Generic Character. Equivalve, orbicular, convex or lenticular, subequilateral, 

 closed, thick and strong ; externally smooth or ornamented with radiating striae or 

 costse. Hinge teeth small, numerous, forming an arched or curved line, central 

 denticles becoming obsolete in old shells. Ligament external, attached to a grooved 

 area in each valve, with distant beaks. Impressions of the adductors two, lateral 

 and strongly marked, that by the mantle entire, or without a sinus. 



* Etym. The diminutive of Pecten. 



