392 Gould's Descriptions of Shells 
presso ad apicem rotundato, margine dorsali declivo, rectiusculo, costà submar- 
ginali munito. 
Shell milk white, fragile, concentrically undulated, ovate, ventri- 
cose; beaks a little anterior, gaping widely behind; the undula- 
tions ending abruptly at a posterior submarginal ridge; the undu- 
lated portion is also minutely corrugated, the wrinkles running from 
the beaks towards the margin; an impressed area in front of the 
beaks is also destitute of waves ; anterior half broadly rounded and 
tumid ; posterior half narrowed, compressed and acutely rounded, 
the superior margin being a rectilinear slope. 
Length two and a half inches ; breadth one and one fourth inches ; 
height two inches. 
Inhabits La Paz, Lower California. Lieut. Green. 
Differs from L. canaliculata Say, of S. Carolina, in its strictly 
ovate form, in having the beaks anterior instead of posterior ; in 
having the posterior half less compressed and more gaping, and in 
the acute ridge at which the undulations terminate. While in size, 
texture, color, and surface they are remarkably similar, they are 
quite reversed in their proportions. 
AMPHIDESMA FLAVESCENS. 
Amphidesma flavescens, Gould ; Proc. B. S. N. H. Nov., 1851, IV. 89. 
T. subrotunda, compressa, dilut® aurantia demum albida et epidermide fusci, 
tenui induta, striis concentricis lamellosis, lineis radiantibus decussatis mM- 
i i i ud elevatis ; 
tincta punctis lucidis notata; sinu siphonali spatulatá, striis crebris divergen- 
tibus insculptá; foveà ligamentali profundá, productà; dente laterali antico 
apici approximato. 
Shell subrotund, nearly equilateral, moderately convex, anterior 
end broadly rounded ; posterior dorsal margin straight and sloping» 
the end less broadly rounded ; posterior umbonal slope with an indis- 
tinet ridge ; beaks acute, not prominent, anterior dorsal area exco" 
vated ; posterior dorsal edge long lanceolate, concave, bounded by 
a distinct angle; surface pale orange near the beaks, becoming 
dingy white at the older stages, and covered by a dirty greenish 
epidermis; marked by concentric lamellar striæ, crossed by fine 
