﻿IGO BRITISH FOSSIL TRIGONI^. 



even to the apex ; that of the right valve is much larger than the other, and in specimens 

 approaching to the adult condition its indentations over the posteal half degenerate into 

 irregular large transverse plications which are sometimes united to the posteal extre- 

 mities of the costae ; the inner carina, although small, is prominent and nodose ; the 

 intercarinal costellae are irregular, unequal, and variable in luuBber; usually those of the 

 right valve are the larger, more especially adjacent to the post-carinal furrow ; occasion- 

 ally the median carina divides into two costellaj and then loses its prominence. The 

 costa) (about 25 in adult forms) are large but somewhat flattened, and (excepting in the 

 young condition) are widely separated ; they have an undulation near to the anterior 

 border, and also turn slightly downwards near to their posteal extremities, where also in 

 adult forms the few last-formed costse become somewhat irregular and broken. The 

 lines of growth are prominent over the whole of the shell, and assume the form of 

 irregular plications when specimens are of advanced growth ; they also sometimes render 

 the anteal portions of the costae granulated. 



Fully developed specimens of T. inontlifera are probably the largest examples of the 

 section ; occasionally Dorsetsliire shells have the granulated epidermal tegument pre- 

 served over the greater portion of the surface, the lines of granules are distinctly 

 separated in the rows, and are sometimes perceptible even to the unaided vision. The 

 convexity of the valves varies considerably in specimens of the same size from the same 

 formation and locality even when there is no appearance of distortion or compression. 

 The following measurements refer to two of our specimens ; these are only of medium 

 size compared with some others which have scarcely less than twice their linear dimen- 

 sions ; the latter, however, are usually more or less compressed, and are therefore 

 unsuitable for comparison. 



r Length measured upon the marginal carina, 3^ inches. 

 No. 1. ■< ,, across the valve at right angles to the carina, 2 inches. 



^ ,, across the united valves, 1-^ inch. 



/-Length upon the marginal carina, 4 inches. 

 No. 2. < ,, across at right angles to the carina, 2^5- inches. 



^ ,, across the united valves to the carina, 1^ inch. 

 The internal mould is smooth, but has a slight rib, indicative of the position of the 

 marginal carina ; the hinge-processes are very large, they project considerably, cor- 

 responding with the great breadth of the escutcheon, and considerable incurvation of the 

 umbones. 



The specimen figured by Agassiz (' Trigonies,' tab. 3, figs. 4 — G), from the terrain a 

 chailles or Oxford Oolite, has the general aspect of a dwarfed variety of the fine species 

 which attains such large dimensions in the lower portion of the Kimmeridge Clay of 

 Wiltshire and Dorsetshire. 



Affmiiive and Distinctive Characters. — The very considerable convexity of the valves 

 inesially, the ornamentation of the escutcheon, the prominence and general narrow 



