MUSEUM OF COMPARATIVE ZOOLOGY. 243 
treme form of reticulata. In the absence of specimens for comparison, how- 
ever, the question cannot be fairly settled. It is a shallow-water species, and 
the material obtained by the “ Blake” was all immature or dead. 
Arca Adamsi SHUTTLEWORTH. 
Arca lactea C. B. Adams, non Linné. 
Arca celata Conrad, Tert. Form. U. S., p. 61, pl. xxxii. fig. 2 (1844), not of Reeve. 
Dredged by Sigsbee off Havana in 80 fms. ; and at Station 220, near Santa 
Lucia, in 116 fms. 
This species is very common in shallow water throughout the West Indies, 
and extends northward nearly or quite to Cape Hatteras. Its simulated ribs 
of trailing blisters give it a remarkably similar appearance to Arca lactea, which — 
however has real ribs. There is a dwarf, very short squarish variety, which 
from its greater proportional diameter (though not otherwise different) would 
at first be separated as distinct, and which may be called Arca Adamsi var. 
Conradiana. 
Arca Noe Linxné. 
Arca barbadensis D’Orbigny, II. p. 321, as of Petiver. 
Arca occidentalis Philippi, and C. B. Adams. 
A valve of this common form appears in the collection from Charlotte Harbor, 
Florida, in 13 fms. It is common in shallow water throughout the West Indies. 
It is possible that the Antillean form may be separable from that of the Medi- 
terranean, but I have not been able to examine the matter critically as yet. 
Arca umbonata Lamarck. 
What appears to be a dead valve of this species was dredged at Station 282, 
near Barbados, in 154 fms. It may have been disgorged by a fish. 
' Arca ectocomata, n. s. 
Plate VI. Figs. 9, 10. 
Shell white, compressed, elongate, equivalve, very inequilateral ; covered with 
a long, soft, silky red-brown epidermis projecting in ribbon-like strips, which 
may be broken up into narrow flat filaments, and project especially at the 
lower posterior angle of the shell ; valves gaping slightly for the large stout 
byssus ; external sculpture of narrow, somewhat irregular, minutely nodulous 
concentric waves ; the interspaces sparsely radiately striate; these striz and 
little nodules correspond to thickened radii in the ribbon-like epidermis which 
are seated on them ; these radii in old shells remain after the flattened web 
which connected them is worn away, and so give to the older shells the aspect 
