﻿180 BRITISH FOSSIL TRIGONIiE. 



bipartite, wilh two elevated, strongly indented, bounding carinae ; the upper half of the 

 area or portion adjacent to the inner carina has much concavity, with six or seven small 

 knotted costellae which take the same direction as the carina, and disappear upon the 

 posteal half of the area, obliterated by the large irregular plications of growth, M'hich 

 there cross the area and effiice the surface ornaments ; the other portion adjacent to the 

 marginal carina is slightly convex, with three or four knotted costelkc which are larger 

 than those of the upper or concave space ; there is no distinct median carina, but the 

 uj)per or inner costellse acquire the semblance of a median carina from its prominent 

 position adjacent to the more depressed concave space. The marginal carina is narrow, 

 elevated and ridge-like upon the umbo, becoming more rounded and depressed posteally, 

 it is crossed by large irregular sub-nodose plications, which are eflPaced posteally by the 

 plications of growth ; the inner carina is also prominent near to the umbo, it is occupied 

 by a line of unequal, irregular, nodose varices formed by the lines of growth. The ante- 

 carinal groove of the left valve is narrow, and similar in character to the post-carinal 

 groove of the right valve. The other portion of the surface has about eighteen rows of 

 elevated narrow plain costfe, all of which originate at the anterior border, their direction 

 is at first horizontal upon the depressed anterior sides, but with the curvature of the 

 valve, each bends obliquely downwards towards the marginal carina until it arrives at the 

 groove of the left valve, when its extremity becomes suddenly attenuated and passes 

 downwards to the extremitv of the costa next in succession : in the right valve the 

 attenuated extremities of the costs) touch the marginal carina. In fully developed shells 

 the two or three last formed costse ^have less obliquity, and follow more nearly the 

 direction of the lower border. 



The lines of growth have great prominence ; they produce reticulations where they 

 cross the costellas of the area, and upon the sides of the valves they sometimes even 

 break the continuity of the costa;, giving to them a wrinkled or broken aspect. 



One other feature, altogether unusual in this genus, and which imparts a distinctive 

 character to the present section, consists in the presence of a small, distinct byssal 

 aperture, with a slightly thickened margin upon the anterior border of the valve, a feature 

 which pertains only to examples of fully developed growth. 



In fully developed specimens the length of the marginal carina is equal to twice the 

 diameter of the united valves, and is somewhat greater than twice the diameter of a valve 

 measured at right angles to the marginal carina. 



It is distinguished from all others of the costatse by the unusually large escutcheon, 

 by the great prominence of the bounding carinae to the area, by the great obliquity and 

 straightness of the costse; lastly, by the presence of the byssal aperture. 



English Neocomian specimens, when of full size and retaining the test, are usually 

 defective at the posteal portion of the area and escutcheon, the siphonal border and 

 adjacent thin portions of the shell having been broken away retain the sharp fractured 

 edges, a condition which indicates that the moUusk was destroyed suddenly, and probably 



