32 



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 (PI. 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 rcc- 

 tilineari, hiantc ; area cardinal! lata, valida ; fovea ligamentalis minime profunda', 

 costa transversali suffulta. 



Shell obliquely ovate, triangular, solid, dirty while, with eighteen 

 radiating ribs, covered with long semi-erect muricated scales, long- 

 est at the posterior margin ; beaks acute, divaricate from each 

 other, slightly auriculule, the margin of the wing forming a right 

 angle wiih the posterior margin ; posterior and ventral margins 

 continuous ; anterior margin rectilinear, inflected, so as to form a 

 broad anterior face, with muriculate striaB, broadly gaping nearly 

 half the length, the edges everted at the upper portion so as to form 

 conspicuous nymphse ; hinge area broad and firm ; ligament pit 

 shallow, with a strong transverse rib below it. Interior shining, 

 conspicuously indented by the ribs. 



Length one and a half inches; height one and one eighth inches ; 

 brendth 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 scales 

 are more crowded, twice as long, inflexed at their points, and much 

 more decided at the dorsal margin ; there are no transverse stria? 

 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. 



