BIVALVIA. 39 



Spec. Char. Testa suborbiculari, cequilaterali, subaquivalvi, radiatd, radiis 14 

 18 sulcatis, plurimum tripartitis, squamoso-denticulatis ; auriculis incequalibus. 



Shell suborbicular, equilateral, slightly inequivalved, with 14 18 large and elevated 

 rays for the most part, one large, with a small one on each side, strongly imbricated 

 ears, unequal, rayed, and squamose. 



Diameter, \\ inch. 



Locality. Cor. Crag, Sutton, Ramsholt, Sudbourn, Gedgrave. 



Red Crag, Sutton, Newbourn, Bawdsey, Walton Naze. 



This is by no means rare in the Coralline, but rather more so in the Red Crag ; it 

 is, however, much less abundant than opercularis, though it may generally be 

 obtained in those localities in which the shells of either Formation are met with in a 

 good state of preservation. 



The valves of this species may be described as equal in size, at least, as far as can 

 be determined by the disconnected valves, which appear to present a similar amount 

 of convexity, although a specimen of the left valve may occasionally be seen a little 

 more tumid or deeper than the right. The rays are fewer than in P. opercularis, never 

 exceeding eighteen, while they have sometimes not more than fourteen, but the general 

 amount is the intermediate number ; these are tripartite, and about an equal width with 

 the spaces between them ; the centre ray is much larger than the one on each side of 

 it, and is covered with large and prominent squamae or imbrications, while the smaller 

 rays are also scabrous, but have more numerous and smaller squamae, not correspond- 

 ing with those upon the larger ray as if they were not formed at the same time by the 

 reflexed edge of the mantle. The spaces between the rays in the young shell are 

 naked or free from longitudinal striae, which, however, become ornamented as the 

 shell increases with from one to three rough and radiating scabrous lines. The 

 length of the shell in most specimens is equal to the height, but when it deviates 

 from this regularity it is in the direction from the anterior to the posterior side, which 

 becomes greater than from the umbo to the ventral margin. Its most distinguishing 

 characters appear to be the inequality of the auricles, the anterior one being much 

 larger than the posterior, particularly in its young state, and much exceeds the 

 differences observable in P. opercularis. The spread of the auricles at the liga- 

 mental edge equals 3-5ths the diameter of the shell, while in some small specimens 

 the hinge area is comparatively larger. On the right valve beneath the anterior 

 auricle, as in the last species, are some prominent denticulations, and the exterior 

 of the shell is also ornamented with fine divaricating striae, visible only with the aid 

 of a magnifier. 



One variety of what I have considered as the young of this species, is rather 

 peculiarly ornamented on the left valve, having every third ray more especially 

 covered with large and elevated imbrications, while the two intermediate ones are 

 nearly smooth upon the outer edge (t. vi, fig. 3). 



