MAZATLAN BIVALVES 137 
187. ARcA EMARGINATA, Sow. 
Proc. Zool. Soc. 1833, p. 20.—Miull. Syn. Test. Viv. p. 180.— 
Rve. Conch. Ic. pl. 4, f. 26.— Hanl. Descr. Cat. p. 161, pl. 18, 
f. 60.—B. M. Cat. D’Orb. Moll. p. 81, no. 723.—C. B. Ad. 
Pan. Shells, p. 258, no. 415. 
Although classed by Sby. among the equivalves, he rightly 
describes it as inequivalve, the posterior ventral part consider- 
ably overlapping, even in young shells. Umbos separated by 
a narrow triangular area. Ligament in very young shells only 
adhering posteriorly: afterwards filling a triangular space to 
the beaks; when adult displaying also a thin film on the 
anterior portion. Teeth strong, not very numerous, both rows 
forming a regular, slightly curved line, not reaching into the 
wing. Emargination not developed in young shells, variable 
in adult. Very youn®shell smooth: afterwards beautifully 
coloured with irregular dark chocolate undulating bands, and 
an interior spot radiating from the umbos. The smallest 
specimen is ‘03 across: the largest (valve) long. ‘5, lat. °9, 
alt. °35. 
Hab.—Atacamas, Real Llejos, Xipixapi, Panama; in sandy 
mud, 6-8 fm.; Cuming.—Do.; D Orbigny— Panama, very 
rare; C. B. Adams.—Gulf of California, Mus. Cuming.— 
Mazatlan; young on Spondylus ealcifer, adult on Murex 
nigritus, extremely rare; L’pool Col. 
Tablet 640 contains 4 sp. young.—641, the largest valve. 
188. Arca————, jun., sp. ind. 
Comp. Byssoarcea alternata, Sow. Proc. Zool. Soc. 1833, p. 17.— 
(Arca a.) C. B. Ad. Pan. Shells, p. 257, no. 413. (Hab. 
W. Columbia, Cuming: Panama, C. B. Adams: Mazatlan, 
Col. Jewett.) 
Tablet 642 contains 2 small opposite valves, the largest °06 
across, remarkable for the structure of the hinge, which is 
bounded not by one but by two lines, one of which radiates 
from the umbo, and between which probably the ligament is 
fixed. Outside are concentric irregular lines near the umbo, 
afterwards about 40 well rounded regular ribs, with equal 
interstices and strong plications within. It is clearly a young 
shell; too young to determine even the generic section with 
confidence. 
Hab.—Mazatlan ; off Spondylus calcifer, extremely rare : 
T’pool Col. 
