manner: it is probable that these vessels coalesced and formed a 
lining for the spines, for the better circulation of the water. Their 
arrangement is exhibited in diagrams Nos. 15 and 16. The lower 
surface of the ventral valve d by the same beautifully 
reticulated a as the upper; the whole surface, except 
muscular, impr ere found, unless a semilunar depression 
found on the ventral aspect‘of the dorsal valve, near the umbo, in 
h. 
Comparisons and Differences.—In ge eral form, this. species re- 
sembles somewhat the P. Altonensis of Drs. Norwood and Prat- 
ten, but, independently of its belonging to a different group, (the 
semireticulati,) it possesses the marginal ring and other charac- 
cies except the P. marginalis of De Koninck, from which it is 
separated by its well marked sinus, the saliency of its ribs on the 
several other minor distinctions, which show that, though allied to 
the marginalis in its cingulum, it differs more than specifically in 
many other important points. . 
This shell resembles in some respects the P. splendens of Nor- 
wood & Pratten, but differs in being less transverse, in the short- 
ness and evenness of its ‘hinge line, in the absence of the two 
tubes on the auricular expansions, the greater distinctness of the 
concentric furrows on the visceral disk, the less pr d depth 
of the dorsal sinus, and the'distinct terminal ring at the base-of the 
dorsal valve. The ventral valve of the P. splendens has a cingus 
