﻿192 



BRITISH FOSSIL TRIGONLE. 



Trigonia cymba, Cont. PI. XXXVIII, fig. 1 (Clavellat^e) . 



Tkigonia ctmba, Contejean. Etude de l'Etage Kimmeridien dans les en v. de Mont- 

 beliard, et dans le Jura de France et Angleterre, 

 Extr. Mem. de la Soc. d'emulation du Doubs, pi. xiv, 

 figs. 1, 2, 1859. 



One of the Clavellata, remarkable for the considerable elongation of the valves 

 posteally, for the small curvature of the rows of costse which are nearly horizontal, for 

 their inconspicuous tubercles, and for the small development of the ornaments upon the 

 valves generally. 



The general figure has some resemblance to the more lengthened forms of the 

 Plioladomya ; the umbones are large, elevated, and nearly erect, placed within the 

 anterior third of the valves ; the anteal portion of the shell has considerable convexity, 

 the posteal and more lengthened portion is comparatively much depressed. The escut- 

 cheon is lengthened, narrow, flattened, and slightly depressed. The area is narrow, 

 bounded upon each side by a row of minute tubercles over the anteal or umbonal half 

 of its length ; over the same portion there is also a distinct median furrow and delicate 

 transverse plications ; these ornaments disappear about the middle of the valve ; the 

 posteal half of the area has only transverse rugae which are not very strongly defined ; 

 it is also much depressed. The other portion of the shell has rows of clavellated costse 

 about fifteen or sixteen in number, small, nearly horizontal, or coinciding in their 

 direction with the lines of growth ; anteally they do not extend to the border of the 

 valve. The tubercles in the rows are small, nearly equal, compressed, little elevated, 

 and near to the lower border they become small scabrous elevations. 



Length of the specimen figured 4 \ inches ; height 2 inches ; convexity of the single 

 valve 7 lines. 



The imperfect example herewith figured was obtained by J. C. Mansel Pleydell, 

 Esq., in Portland Sand of the cliffs in Kimmeridge Bay, Dorsetshire. The minuteness 

 and delicacy with which the characters of the surface have been preserved leave little 

 cause to regret the absence of the test, but the surface ornaments have much less promi- 

 nence than in the fine specimen figured by M. Contejean, from Mont. Beliard, at which 

 locality he states that it is abundant. 



The unusually lengthened and much curved figure, together with the numerous, 

 small, nearly horizontal rows of costse, and inconspicuous closely set tubercles, serve to 

 separate it from other clavellated Trigonim of the Upper Oolites. The convexity 

 anteally is considerable and contrasts with the lengthened and depressed posteal portion 

 of the valves ; these comparisons refer more especially to T. Pellati, Mun., T. Cottaldi, 

 Mun., T. Alina, Cont., and T. muricata, Goldf. 



