﻿142 



BRITISH FOSSIL TRIGONE. 



the escutcheon equal to about two fifths of the entire valve • it has a faintly defined, 

 mesial, oblique depression, and is covered by a very numerous and delicate series of 

 obliquely curved scabrous costellae which nearly disappear upon its posteal portion ; the 

 escutcheon is of moderate breadth, separated from the area only by the border of its 

 concave surface, and by the greater prominence of its costellae, which are continuations 

 of those upon the area ; these form a slight angle at the border of the escutcheon and 

 traverse it in a direction directly transverse to its superior border, which is somewhat 

 raised. Upon the divisional line of the valve the extremities of the costellae are united to 

 the superior or attenuated extremities of the pallia! costae, forming with them acute 

 angles. The rows of costae, which are very numerous and small, are curved obliquely 

 downwards ; their upper borders form projecting obtuse nodosities, the extremities of a 

 multitude of perpendicular regular plications or little pillars which cross the costae. 

 Fully developed specimens have the plications slightly waved and somewhat irregular 

 near to the pallial border, where the extremities of the costse become more distant. No 

 specimen altogether entire has come to my notice. 



It will be observed that the specimens figured on Plate XXV constitute two 

 varieties, which pertain to different localities and beds of the Upper Greensand. The 

 left-hand figure (8) is from the Chloritic Sand at Chardstock ; it also occurs at Dunscomb 

 cliff's eastward of Sidmouth, and near to Axmouth in the Chloritic marly beds ; the costae 

 are smaller than in the other variety, and more closely arranged ; they also form a much 

 smaller angle with the costellae upon the area ; their upper borders have also less 

 prominence. Specimens are in the collection of the Royal School of Mines, in 

 Mr. C. J. A. Meyer's collection, and in my own cabinet. The right-hand figure (9) is 

 apparently less rare, and has been obtained by Mr. Vicary in a pebbly bed overlying 

 the Greensand at Great Haldon. No specimen altogether perfect has been obtained. 



This, which I arrange as the typical form, has the costae somewhat larger anteally and 

 more widely separated; they become much attenuated at the divisional angle of the 

 valve, and form considerable angles with the costellae upon the area; the intercostal 

 perpendicular plications are also larger and more conspicuous. Some specimens are 

 delicately silicified, and exhibit the most minute surface ornaments with great 

 clearness. It is intended to give additional figures of the typical form in a future 

 plate. 



Affinities and Differences. Defective specimens of the variety with the smaller costae 

 were at first mistaken for T. tenuisulcata, Dujardin (' Mem. Soc. Geol. de France,' vol. ii, 

 pi. 15, fig. 11) ; the examination of more satisfactory examples has led to their separation 

 from Dujardin's species, which has the costae more minute, more closely arranged, and 

 peculiarly straight near to the pallial border ; the area and escutcheon are also smaller 

 and less expanded than in the British species. 



T. LamarcJcii, Matheron (' Catal. des Corps org. foss. du Depart, des Bouches du 

 Rhone,' pi. 22, figs. 5 — 7) has affinities with our species in the general figure and the 



