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 Nucula inhabiting the Northern 

 Hemisphere, or of any of our well-known Tertiary species. Two fossils found in the 

 Cretaceous Formations (N. bivirgata 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 striae, 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 Lucints, 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. 



