88 



cence. The epidermis is prolonged posteriorly, much as in 

 specimens of Mya truncata. There seem to be no accessory 

 pieces at the hinge, though the hinge margin is reflected into a 

 broad plate on each side so as to answer the same end. 



Petricola bulbosa. T. solida, ovato-cuneata, postice hian- 

 tula, costis radiantibus numerosis parvis compressis ornata, ad 

 interstitiis concentrice striatis ; apicibus ad trientem anteriorem, 

 tnmidis ; latere antico subgloboso ; latere postico subtriangulari ; 

 compresso, ad apicem truncate ; margine ventrali coarctato : 

 intus livida, cicatricibus calcareis : cardo dente unico ad valvam 

 dextram subulate, recurvo instructus. Long. \^ ; lat. f ; alt. ^ 

 poll. Inhabits Guaymas. Lieut. Greene. 



This is the analogue of P. rocellaria, Lk. which it very 

 closely resembles. The posterior end declines more rapidly ; 

 the radiating folds are much more delicate. It grows larger. 

 The dark leaden hue of the interior is not constant, nor are the 

 cicatrices always chalky. 



Mactra nasuta. T. solida, transversa, ovato-cuneata, albida 

 epidermide stramineo postice incrassato et fuscato induta ; ver- 

 tice fere mediano, acuto ; latere antico angustato, compresso, 

 subadscendente ; latere postico dilatato, truncate, hiante ; area 

 dorsali postica lanceolata, excavata : intus polita, Candida ; fovea 

 ligamentali perobliqua ; dente V-formi tenui, elongate ; dentibus 

 lateralibus crassis. Long. 3^ ; lat. 1 ; alt. 2^ poll. Mazatlan, 

 Lieut. Gree7i ; San Pedro, Moj. Rich. 



The analogue of 3L Brasiliana., distinguished by the poste- 

 rior position of the beaks, and the more attenuated form of the 

 anterior half caused by the concave outline of the dorsal margin. 

 The parts composing the hinge are more oblique. 



Mactra mendica. T. ovato-triangularis, compressa, concen- 

 trice striata, epidermide rufo induta ; apicibus antemedianis, 

 acutis, incurvatis ; margine dorsali utrinque cito declivi ; latere 

 antico cito rotundato ; latere postico triangular!, desuper viso 

 anguste cordiformi profunde striate: intus lactea ; sinu siphonali 

 parvulo ; fovea ligamentali retrorsum profunde effossa ; dente 

 V-formi parvo ; dentibus lateralibus conspicuis. Long. 1^ ; lat. 

 ^ ; alt. I poll. Mazatlan, Lieut. Green. 



