﻿COSTAT^E. 



171 



the shell ; its surface is very delicately reticulated ; it is bipartite, each portion having very 

 small longitudinal costellae, one of which is sometimes prominent, forming a small median 

 carina ; in other examples, and more especially in the right valve, there is no distinct 

 median carina. The bounding carina? are very small upon both the valves, but the 

 marginal carina is always distinct; it is minutely, transversely plicated and the ante- 

 carinal groove of the left valve is only slightly defined ; the umbonal portion of the carina 

 has a considerable and graceful curvature. The inner carina is commonly only indistinct ; 

 it is one of the reticulated costellae of the area slightly separated from the more depressed 

 surface of the escutcheon. 



The other, and by much the larger portion of the shell, has the rows of costae small 

 and numerous; their edges are acute, little elevated, uniform in character; their number 

 in fully developed specimens varies from 24 to upwards of 36, depending upon the more 

 close or distant arrangement of the rows ; the first formed or umbonal rows have their 

 general direction obliquely downwards posteally ; the rows subsequently formed are 

 directed more horizontally, but curve upwards anteally, where they form a well-marked 

 undulation ; near to the lower border they take the direction of the lines of growth or 

 conform to the figure of the border ; the left valves sometimes have the posteal extremities 

 of the costse, each forming a short, downward prolongation ; the right valve has several of 

 the last-formed costa? passing across the marginal carina as so many plications. Well- 

 preserved specimens have the costse crowded with minute perpendicular lines of epidermal 

 granules. 



Young examples when less than fifteen lines in length are much depressed, the costse 

 have no distinct anteal undulation, and the several features which characterise the fully 

 developed shell are scarcely perceptible. 



A large, gracefully curved, and transversely lengthened form, remarkable for the large 

 curvature of the small marginal carina, the produced and attenuated posteal extremity, the 

 narrow, excavated, and minutely reticulated area, the considerable angle which it forms 

 with the costal portion of the surface, the largeness of that surface and the rounded and 

 produced anteal side ; the combination of these characters will usually readily distinguish 

 it from others of the costatce. 



Judging of T. Cassiope from our figures only, it might be imagined that the species 

 is divisible into varieties ; a more extended knowledge of this Cornbrash form will lead to 

 the more correct inference that these figures represent nearly the extremes of variation 

 in opposite directions ; figure 1 exemplifies the more wide and depressed, and figure 

 2 the more narrow and inflated forms. The greater number of specimens will be found 

 to approximate to figure 4 ; there is, however, so much variability both in the outline of the 

 valves, in their convexity, and in the size and number of the costse, that taken in combina- 

 tion with the frequent and, indeed, usual compression or distortion of some portion of 

 the shell, it is scarcely possible to find any two specimens having any close agreement 

 with each other. Occasionally a valve occurs even more inflated than the narrow 



