RECENT AND TERTIARY SPECIES OF LEDA AND NUCULA. 177 
§ 4. Descriptions of some Species of Westindian 
Molluska. 
I take advantage of the present opportunity to give sketches 
and descriptions of five shells, two of which have been pre- 
viously described, but not figured; the other three are new. 
Of the five, four are inhabitants of the Gulf of Paria, and 
the fifth is a small but curious shell from the miocene of 
Jamaica. Metula Untea and Periploma orbicularis were 
dredged in the Gulf of Paria by Mr. W. 0. Crosby, of the 
Boston Society of Natural History. Mr. Crosby discovered 
not only the species mentioned in this communication as 
dredged by him, but several others which he kindly com- 
municated to me, and which are of remarkable interest as 
being either identical with, or nearly allied to, fossils of the 
Westindian miocene. As the specimens referred to are all 
dead shells, devoid of colour, it has occurred to me as a 
possibility that they might have been derived from some 
tertiary bed at the bottom of the Gulf. An hypothesis like 
this, however, would be at once refuted by the finding of a 
single living example. 
1. Periploma orbicularis n. sp. (PI. vii., f. 13). 
Suborbicular, subeqnilateral, somewhat compressed, thin, 
nacreous internally, shagreened externally by minute rugo- 
sities disposed in radiating lines, and covered witli a thin 
epidermis; anterior side regularly rounded; posterior side 
somewhat produced, sinnate, and obliquely truncate ; umbones 
prominent, fissured. Height 25 mill., length 30 mill. 
Resembles the Perijjloma compressa of D’Orbigny (Voy. 
Amer. Merid., p. 514, pi. lxxviii., f. 19, 20), but having a 
greater relative height from the umbo to the margin, is more 
