211 



Long. 3^; lat. 1 ; alt. 2\ poll. Mazatlan, Lieut. Green; San Pedro, 

 Maj. Rich. 



[This species agrees so nearly with M. falcata, Gel., that until a 

 direct comparison is made it may be considered as synonymous. M. 

 falcata, however, was found at Puget's Sound.] 



Mactra mendica [Gnathodon trigonum, Petit ; Journ. Conchyl- 

 iol., May, 1853]. 



Lutraria undulata. T. Candida, papyracea, ovata, postiee 

 hians, nisi prope marginem posticum concentrice undata, coneinne 

 rugosa ; apicibus vix anticis ; latere antico rotundato ; latere postico 

 angustato, compresso, ad apicem rotundato ; margine dorsali declivo, 

 rectiusculo, costa submarginali munito. 



Long. 2£; alt. 2 ; lat. 1| poll. Inhabits La Paz, Lower California, 

 Lieut. Green. 



Differs from Z. canaliculata, Say, of South Carolina, in its strictly 

 ovate form, in having the beaks anterior, in having the posterior half 

 less compressed and more gaping, and in the acute ridge bordering 

 the posterior margin. [The figure of Mactra elegans, in the Tanker- 

 ville Catalogue may have been intended for the same shell.] 



Lutraria ventricosa. T. tenuis, fragilis, ventricosa, ovato- 

 cuneata, alba, epidermide strarninea rugis remotis radiata induta ; 

 apicibus vix anticis, tumidis; latere antico angustato, compresso; 

 latere postico dilatato ; facie dorsali valde declivi, angulo obtuso 

 limitata, aream latam concavam effbrmante linea submarginali par- 

 titam : intus alba, punctis nitidis notata. 



Long. 3j ; lat. If ; alt. 3 poll. Inhabits Mazatlan, Lieut. Green. 



This would at first sight seem identical with L. carinata, from the 

 coast of Carolina and Florida. But it is more ventricose, less solid, 

 the beak more arching, the region of the posterior margin excavated, 

 and the posterior umbonal ridge obtuse and destitute of a crest. A 

 small African species (Mactra albida) is a miniature of this shell. 

 [It has been thought, by some, to be M. exoleta, Gray (1837) ; but I 

 think it is not.] 



Amphidesma [Semele] flavescens. T. subrotunda, conipressa, 

 striis concentricis lamellosis decussatis insculpta, aurantia demum 

 albida et epidermide fusca induta; apicibus medianis, acutis, baud 

 elevatis; area dorsali antica excavatii, postica lanceolata, concava, 

 linea limitata ; intus flavo-tineta punctis nitidis notata ; sinu siphonali 

 spatulato, striis crebris divergentibus insculpta ; fovea Hgamentali 

 profunda, elongata ; dente laterali antico ad apicem approximate. 



Long. 2|; alt. 21; lat. 1± poll. San Diego. Lieut. Green. 



Usually found about half the above size ; the concentric lamella; 

 become worn off and more irregular towards the margin. The in- 



