from the Gulf of California, ete. 405 
the byssal sinus is deeper and much more acute, making a trape- 
zoidal auricle. The cauda is much shorter in A. tarentina and the 
wing much more oblique. It most nearly resembles a Florida 
species, but in that the byssal angle is obtuse, the auricle triangular, 
and the cauda shorter. 
LIMA TETRICA (PL. XVI. Fig.6.) 
Lima tetrica, Gould; Proc. B. S. N. H., Nov. 1851. IV. 93. 
T. oblique ovato-triangularis, solida, albida, costis radiantibus ad 18, squamis 
semi-erectis posticis longioribus armatis horrescens; apicibus acutis, remotis, 
muriculatis ; marginibus posticis et ventralibus continuis; margine antico rec- 
tilineari, hiante ; areá cardinali latá, validà ; fovea ligamentalis minime profunda, 
costa transversali suffulta. 
Shell obliquely ovate, triangular, solid, dirty white, with eighteen 
radiating ribs, covered with long semi-erect muricated scales, long- 
€st at the posterior margin; beaks acute, divaricate from each 
other, slightly auriculate, the margin of the wing forming a right 
angle with the posterior margin; posterior and ventral margins 
continuous ; anterior margin rectilinear, inflected, so as to form a 
broad anterior face, with muriculate striz, broadly gaping nearly 
half the length, the edges everted at the upper portion so as to form 
conspicuous nymphe ; hinge area broad and firm; ligament ue 
shallow, with a strong transverse rib below it. Interior shining, 
conspicuously indented by the ribs. : 
ngth one and a half inches; height one and one eighth inches ; 
breadth half an inch. 
Inhabits La Paz, Gulf of California. Maj. Rich. 
Compared with L. squamosa, it is narrower, and more compressed ; 
the posterior and ventral margin is less broadly rounded ; the ne 
are more crowded, twice as long, inflexed at their points, and muc 
more decided at the dorsal margin; there are no transverse striæ 
on the dorsal area. The strong brace or rib in front of the liga- 
ment pit, by which a deep cavity is formed at the apex, is remark- 
able, but possibly accidental. 
