﻿QUADRATE. 107 



like figure. This, however, is not constant, and not unfrequently the nodes are ovate or 

 shghtly oblong, or in other instances the marginal carina has its nodes unequal in size 

 and irregular in figure ; all the rows become evanescent upon the posteal half of the valve, 

 where the area is occupied by large, irregular, knotted, transverse plications ; the portion 

 of the area near to the umbo has regular transverse ridges, which are united to the 

 carinal nodes ; the whole of this portion of the area has rows of regular, minute, trans- 

 verse, papillary elevations, more or less distinct, which impart a highly ornamented 

 aspect to the valve. The rows of nodose varices or costse upon the other or pallial 

 portion of the valve commence, as in some other species, with about seven elevated 

 biangulated, ridge-like rows, which pass obliquely downwards from the anterior border to 

 the middle of the costated surface, where they are bent suddenly and pass uninter- 

 ruptedly across the area obliquely, forming small elevated nodes at the marginal and 

 median carinse, and smaller ridges as they are prolonged to the inner carina; the 

 succeeding rows consist of large, closely arranged, rounded varices, curved obliquely 

 downwards from the marginal carina ; their nodes usually increase in size downwards in 

 each row ; as they approach the pallial border they curve upwards in the direction of the 

 lines of growth ; there is some irregularity in the rows of anteal varices which succeed the 

 first-formed plain ridged series ; they form a few^ short supplementary rows, several of 

 which are intercalated with the anteal portions of the oblique rows ; the nodes nearest to 

 the carina are smaller than those of the carina, with which they are also unconformable 

 in number, as they are slightly more numerous than the carinal nodes. Owing to the 

 enlargement of the nodes nearest to the pallial border and their more lengthened figures, 

 the aspect of that portion of the valve is sometimes remarkable, the nodes being so 

 closely placed that their arrangement in rows is scarcely perceptible unless the shell is 

 viewed from the anterior side and at a little distance, when their order becomes more 

 evident. 



In the variety Orbignyana the size and figure of the nodes have commonly 

 great variability and inequality, sometimes even in a single row, so that the anteal or 

 pallial extremities of the rows have the nodes large and ovately lengthened, or in other 

 specimens their anteal extremities are attenuated, their nodes becoming small, cord-like, 

 and indistinct ; both kind of rows have their anteal extremities conformable with the 

 lines of growth or with the pallial border. For the most part the variety of the Perna 

 bed or typical form has the rows of varices and nodes rather smaller and more regular 

 (Plate XXV, fig. 1). The rows are more separated ; the carinal nodes are also more 

 regular and equal, so that the general aspect has little of the confused and crowded nodes 

 so commonly seen in the variety of the crackers. 



Young examples, from five to fifteen lines in length, have the general figure depressed, 

 and are shorter or more quadrate ; the area is also in proportion larger ; the costae or 

 varices form plain ridges without nodes. 



The " Cracker " variety Orbignyana has occurred abundantly at Atherfield, in beds 



