﻿188 BRITISH FOSSIL TRIGONItE. 



Vicary at Great Haldon in tlic pebble-bed which overlies the Blackdovvn and llaldon 

 Greensand, associated with other characteristic Trigonia of the Upper Greensand. 

 Another imperfect specimen was obtained by Mr. Meyer in the Chloritic Marls in the 

 vicinity of Axmouth. It appears to be rare. 



Trigonia dunscombensis, Lye, sp. nov. PI. XL, figs. 5, 6 (GLABRiE). 



Shell subovate or ovately oblong, very convex, umbones antero-mesial, prominent, not 

 recurved ; superior border slightly concave, its posterior extremity curved elliptically with 

 the siphonal border, which is short and curved with the inferior border ; the anterior 

 border is produced and rounded. In the young condition the costse are very closely 

 arranged or almost linear ; they pass horizontally across the valve and are slightly 

 undulated at each of the extremities in a manner resembling T. excentrica, but less 

 pronounced. With advance of growth the costae become narrow, prominent, and much 

 more distantly arranged, their prominence continues in well-preserved specimens even 

 to the lower border ; they disappear posteally, so that about one third of the valve is 

 plain. The valves have no decided arrest of growth, but the lines of growth are irregular 

 and strongly defined. 



The nearest ally is T. excentrica, compared with which the general figure is shorter or 

 less produced posteally ; the convexity is much more considerable ; the eostte are more 

 ^prominent, less excentric anteally, the rows are also more widely separated, more especially 

 upon the middle portion of the valve and near to the lower border, at which part of the 

 valve in T. excentrica the costae are usually evanescent. 



Dimensions of the specimen, PI. XL, fig. 5 : 



Height 21 lines ; length 26 lines ; diameter through the single valve 9 lines. 



The test is of considerable thickness and is often found separated into two layers, of 

 which the inner layer is of much the greater thickness; its surface consists of a series of 

 minute, closely set perpendicular lines, often of unequal size ; they are more or less 

 impressed by distantly arranged concentric lines of growth. 



Numerous specimens have been placed at my disposal by Mr. Meyer, collected by 

 him in the Chloritic Marls of the South Devon Coast, near Sidmouth ; more especially 

 in the clifiFs at Dunscombe, Branscombe, and Beer Head. All the specimens are more 

 or less imperfect or fragmentary and their outlines are rarely preserved entire ; they 

 afford little information concerning the hinge processes or of the interior of the shell ; as, 

 however, large portions of the surface are often very well preserved, the costae can be 

 compared with and separated from the T. excentrica of the Blackdown Greensand. The 

 general very defective condition of the Trigonia Glabrae in the Upper Greensands will 

 account for my having mistaken some of them for T. excentrica as at p. 90. I therefore 



