BIVALVIA. 83 
This species has not, as yet, I believe, been found in any Formation older than the 
Red Crag, in which it does not seem to have been very abundant, but in the Deposits 
of the succeeding period it may be obtained in large numbers, in the portion of that 
Formation resting upon the Red Crag at Chillesford, where the valves are sometimes 
found united, and in their natural position. 
There is no species known with which this is likely to be confounded, as its 
peculiar sculpture differs from that of any recent MVucu/e inhabiting the Northern 
Hemisphere, or of any of our well-known Tertiary species. Two fossils found in the 
Cretaceous Formations (JV. divirgata and ornatissima), possess similar ornament, and a 
recent species has been also obtained from a considerable depth off the Cape of Good 
Hope, which is covered with zig-zag striz, these however have no specific relationship 
with our shell. 
This species, although one of the finest belonging to the genus, has not attained 
quite so great a magnitude as the preceding one, my largest specimen does not 
exceed one inch and an eighth in its transverse or largest diameter, while the 
other has reached to an inch and three eighths. Like most of the shells from 
the Crag, it varies somewhat considerably in its proportional dimensions. In those 
which are most tumid, the diameter is less from the dorsal to the ventral margin, than 
it is in those which are more compressed. The number of teeth are generally from 
sixteen to eighteen on the anterior side, with about ten upon the shorter or posterior 
slope, they are prominent and sharp, of an angular form, and interlocking, and the 
fossette for the ligament is large, projecting inwardly, inclining beneath the dorsal 
edge towards the anterior side, and the umbo is terminal, and somewhat pointed. 
This species is sometimes much thickened internally in aged specimens, forming 
deeply indented impressions by the adductors, which are of a sub-circular form 
inclining to oval, and the marginal impression of the mantle is then ornamented with 
radiations like those in some of the Lucinez, but the margin of the shell is smooth, 
and free from crenulations. 
This pretty shell is ornamented upon the exterior with irregularly divaricating 
striae, which generally, in the young state, have only one series of diverging lines, but 
in the centre part of the older specimens they are more irregular, and become zig-zag, 
with two, three, or more angular points of divergence, the radiations are large and 
rounded, and crossed by transverse or very perceptible lines of growth, and the 
shell when living was probably covered by an epidermis. 
In some specimens the umbo is much eroded, while in others it is quite perfect. 
