19 



presso ad apicem rotundato, margine dorsali declivo, rectiusculo, costa 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 subrnarginal 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. 

 Amphidesmajlavescms, Gould; Proc. B. S. N. H. Nov., 1851, IV. 89. 



T. subrotunda, compressa, dilute aurantia demum albida et epidermide fusca, 

 tenui induta, striis concentricis lamellosis, lineis radiantibus decussatis in- 

 sculpta ; apicibus medianis, acutis, baud elevatis ; area dorsalis antica exca- 

 vata ; area dorsalis postica lanceolate, concava, linea marginata ; intus flavo- 

 tincta punctis lucidis notata ; sinu siphonali spatulata, striis crebris divergen- 

 tibus insculpta ; fovea ligamentali profunda, producta ; dente lateral! antico 

 apici approximato. 



Shell subrolund, 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- 

 tinct ridge ; beaks acute, not prominent, anterior dorsal area exca- 

 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 strise, crossed by fine 



