216 BRITISH FOSSIL TRIGONI^. 



large, acutely ridged plications ; they are somewhat irregular and unequal in their 

 prominence, the surface becoming somewhat depressed at the position of the escutcheon, 

 where the plications are smaller and less distinct ; about eight or nine plications are 

 visible upon the left valve ; the hinge-border slopes obliquely downwards, its length 

 exceeding that of the siphonal border, which is almost perpendicular with the lower 

 border. The dorsal portion of the surface is covered by a series of linear, depressed, 

 horizontal costse ; their size is unequal in different specimens and irregular, sometimes 

 even upon the same valve ; they do not enlarge posteally, and disappear in the ante-carinal 

 groove ; nearly fifty of these linear costae may be distinguished. The right valve has its 

 area somewhat more excavated ; its plications are very irregular in prominence and 

 unequal in size, but smaller and more numerous than those of the left valve ; the 

 marginal carina is smaller, it is plicated more or less distinctly upon its lower portion ; 

 its upper portion, which is smooth, divides into two carinse, which continue separate and 

 distinct to the lower border. There is also a third dorsal carina at a little distance 

 anteally to the second carina, defined chiefly by the greater depression of the surface 

 occupied by these three carinas when compared with the general dorsal linear costse of 

 the right valve, which do not differ materially from those of the other valve. 



The shell is of moderate thickness, even near the lower border. The mould does 

 not exhibit any portion of the external ornamentation. Only single valves have been 

 obtained, and the hinge-characters have not been exposed. 



Height of the largest specimen 5 lines ; length 5^ lines. 



The Bere specimens, in common with some Jurassic Conc/iifera having a hard 

 limestone matrix, appear to have had their surfaces covered and their ornaments 

 concealed by the infiltration of a layer of carbonate of lime between the outer surface of 

 the fossil and its matrix. Small portions of this white film-like layer are still visible 

 upon some of the specimens. I did not consider this exterior surface as having formed 

 any portion of the test. 



