﻿168 BRITISH FOSSIL TRIGONI^. 



Tkigonia Mekiani, (POrbigny. Prodrome de Paleont., vol. ii, etage 14, p. 17, 



No. 262. 



— — Buoignier. Paleont. du Dep. de la Mense, p. 265, 1852. 



— — Waagen. Der Jura in Frank. Schw. u. der Scliweig., p. 218, 



1864. 



— CLAVELLATA (misprint ?), PM/jps. Geol. of Oxford, pi. siii, fig. 2, 1871 



(reduced figure). 



— Meriaxi, Phillips. Geology of Yorkshire, 3rd ed., 1875, vol. i, p. 2.50. 



Shell ovately trigonal, very convex ; iimbones produced, pointed, arched inwards and 

 recurved : anterior side produced, its border rounded elliptically with the lower border, 

 which is slightly excavated posteally ; escutcheon comparatively small, depressed, flattened, 

 with its superior border somewhat raised ; its surface has small, closely arranged, delicate 

 oblique plications ; it is well circumscribed by the small nodose varices of the inner 

 carina ; as the hinge-border slopes obliquely downwards and the siphonal border of the 

 area is also oblique, their junction does not form any prominent angle, but the length of 

 the hinge-border exceeds somewhat that of the other. The area is slightly excavated 

 and flattened, rendered distinctly bipartite by the superior or inner half being more 

 depressed by the other portion ; it is bounded by two well-defined small carinse ; the 

 marginal carina is elevated, peculiarly narrow in the left valve, and somewhat larger in 

 the other valve ; it has small inconspicuous plications ; the inner carina forms a border 

 fringed with closely arranged, small nodosities ; there is also a small, flattened, narrow 

 band which represents the median carina ; the intercarinal costelljB are small, numerous, 

 closely arranged and unequal ; they are nearly alike in both the valves ; the entire area 

 has conspicuous, densely placed, transverse lines of growth, which strongly indent the 

 whole of the surface. The sides of the valves have a very numerous series of costa3 (forty 

 or more in advanced growth) ; they are small, somewhat unequal in size, and irregular 

 in their direction ; they form a flexure near to both of their extremities ; the few last 

 formed costse are more imperfect and irregular ; their anteal portions take the direction 

 of the lines of growth and curve upwards anteally, embracing the extremities of the 

 costse previously formed; in the right valve their posteal extremities pass across the 

 marginal carina as so many plications ; in the other valve they terminate at the small 

 ante-carinal groove. The lines of growth are minute and irregularly crowded over the 

 valves generally. 



The defective, irregular figures and partial effacement of the few last-formed costse, 

 indicate the ultimate stage of growth in the life of the Mollusk when the mantle 

 continues to add to the growth of the valves, but ceases to produce surface ornaments. 



The hinge processes are prominent and lengthened ; the posteal cardinal process of the 

 right valve extends horizontally nearly the length of the escutcheon. The internal 

 mould has not been ascertained. 



A large example in my collection has the length of the marginal carina 4| inches ; 



