﻿304 PLAGIOI.A 



Shell dull colored, sulcatc. 

 Surface radiately grooved. 



Rather solid, tawny or pale brown. P. cyrenoides. 



Subsolid, dark colored. P. imbricata 



Surface not radiately grooved. 



Very solid, t^reatly inflated. P. sallei. 



Subsolid. moderately intlated. P. nicaraguensis. 



Subgenus Pr,.\Gior,A s. s. 



Shell solid, surface irregularly concentrically ridged ; epi- 

 dermis smoothish, but here and there wrinkled ; painted with 

 larger and smaller scattered rays, which are generally broken 

 into irregular lunate or squarish blotches ; hinge heavy and 

 strong ; hinge plate wide and flat ; female shell smaller than the 

 male, more inflated, and swollen at post-basal region. 



Animal having the branchiae rounded below, inner the larger, 

 wholly or partly free from the abdominal sac ; marsupium 

 large, projecting far below the inner gills; mantle very thin, 

 dark on its thickened, fringed douible edge ; anal opening with 

 very small jiapillcie. 



PLACior.A siccuKis (Lea). 



Shell subtriangular, solid, subcompressed or scarcely intlat- 

 ed, with a sharply defined up-curved posterior ridge, behind 

 which it is truncated ; region of the beaks and a considerable 

 area of the upper part of the shell compressed ; beak sculp- 

 ture consisting of a few irregular, slightly doubly-looped 

 ridges; beaks curv^cd forward over a small, wide lunule pass- 

 ing back under the hinge, where it is filled with epidermal 

 matter; ligament small, brown; surface wMth irregular, often 

 rude, sometimes almost sulcate, growth lines ; epidermis rather 

 smooth. Init showing fine, wrinkled loops under a glass, tawny 

 or yellowish-green, generally with faint, broken rays, which 

 are made up of dots or lunate or arrow-head markings ; they 

 are sometimes composed of alternately lighter and darker 

 (lashes ; hinge strong, the plate often flattened ; left valve with 

 two triangular, ragged pseudocardinals, a faint anterior third 



