﻿190 



BRITISH FOSSIL TRIGONLE. 



crenulated costae sometimes twenty-five or more in number, small at the divisional angle 

 of the valve ; they enlarge downward, the summits of the rows are everywhere narrow, 

 and delicately crenulated ; their direction is obliquely downwards over the middle 

 portion of the valves ; the first formed or more umbonal rows are curved forwards 

 anteally, becoming attenuated and almost horizontal upon the anteal face of the shell. 



The internal mould is usually seen in the South Devon specimens ; the dividing 

 ridge of the area is strongly defined ; the lower borders of the valves are slightly dentated 

 and the impressions of the costae are more or less visible, resulting from the considerable 

 attenuation of the test, a feature in which it differs from allied species of the aliformis 

 group. 



Dimensions. Length of the largest specimen figured 32 lines. Height 30 lines ; 

 convexity of a single valve 10 lines. 



Affinities. Compared with T. crenulata from Le Mans, our species has the costae 

 usually more prominent, rugose, and less numerous ; the area and escutcheon are more 

 steep, or form a more considerable angle with the other portion of the shell ; the posteal 

 slope in T. crenulata is therefore more expanded and more fully exposed when a valve 

 is placed horizontally and viewed from above ; the median furrow is distinct, (but is 

 without the deeply impressed groove of T. crenulifera ; but the chief distinction consists 

 in the prominent zigzag costellae upon the area and escutcheon which imparts a character- 

 istic aspect to that portion of our shell. 



T. crenulata is exemplified in specimens from Le Mans in the ' Paleontologie 

 Franchise ' of D'Orbigny, vol. 3, pi. 295, which has the area destitute of costellae. 

 The splendid specimen figured by Agassiz, ' Trigonies,' pi. vi, figs. 4, 6, obtained at 

 the same locality, is depicted with very small irregular crenulated transverse costellae 

 upon the umbonal or anteal portion of the area — a varietal character forming some 

 approximation to, but distinct from, the prominent and peculiar costellae of our 

 species. Compared with T. scabra, Lam., the latter form has the costae upon the 

 sides of the valves much larger ; they pass across the area and escutcheon continuously, 

 much reduced in size, but are not interrupted by the elevated boundaries which mark 

 the limits of that portion of the shell; they are therefore without the large irregular or 

 broken costellae of our species. 



The peculiarities of the costellae possess some resemblance to a similar feature 

 exhibited upon the area and escutcheon in one of the Scabra from Bogota, figured and 

 described by D'Orbigny under the name of T. subcrenulala, ' Coquilles foss. de Colombie,' 

 pi. iv, figs. 7, 8 ; the latter shell has the general figure much more inflated, with a much 

 smaller and more concave area, which, however, is altogether without the deeply impressed 

 median groove of T. crenulifera, the junction with the escutcheon is ill defined, the 

 transverse costellae have an undulation not broken by a median groove as in T. 

 crenulifera ; the crenulated costae upon the sides of the valves are very small and widely 

 separated. 



