BIVALVIA. 39 
Spec. Char. Testé suborbiculari, equilaterali, subaquivalvi, radiata, radiis 14 
—18 sulcatis, plurimum tripartitis, squamoso-denticulatis ; auriculis inequalibus. 
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, 14 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 squame or imbrications, while the smaller 
rays are also scabrous, but have more numerous and smaller squame, 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 strize, 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 striz, 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). 
