﻿160 



BRTTISH FOSSIL TRIGONLE. 



Trigonia tenuicosta, Lyc. Plate XXXIII, figs. 7, 8, 9, 9 a. 



Trigonia tenuicosta, Lycett. Trigonias from Iuf. Ool. of the Cotteewolds, Proc. 



Cott. Nat. Club, vol. i, p. 252, pi. ix, fig. 4, 1853. 



— — Morris. Catalogue, p. 229, 1854. 



— — lycett. Cotteswold Hills Handbook, p. 64, 1857. 



— — Judd. Mem. Geol. Surv., Rutland, &c, pp. 153, 170, 281, 



1875. 



Shell ovately trigonal, very convex; umbones elevated, acute, arched inwards, and 

 recurved ; anterior side very short, its border truncated almost perpendicularly, and 

 slightly excavated beneath the umbones ; inferior border short, curved elliptically ; hinge- 

 border sloping obliquely, and forming an obtuse angle with the siphonal border, which is 

 nearly perpendicular and equal in length to the hinge-border. Area large, concave ; its 

 surface forming nearly a right angle with the costated portion of the valve ; it is rendered 

 unequally bipartite by a minute but distinct median carina in each valve ; the superior or 

 inner portion is much depressed and concave ; the entire area has numerous delicate 

 oblique intercarpal costellae, and is bounded by small, minutely indented, distinctly 

 elevated carinas. The marginal carina has its transverse plications very narrow, 

 numerous, and nearly regular ; their number is equal to thrice those of the costse. The 

 escutcheon is wide, heart-shaped, with the valves in contact, and slightly depressed ; its 

 superior border is convex ; its surface is occupied by densely arranged oblique lines of 

 minute granulated lineations. The other portion of the surface has the costse, about 

 twenty-eight in number, narrow, elevated, nearly horizontal, curving upwards anteally, 

 and there forming a sudden undulation, their attenuated extremities meeting the anterior 

 border horizontally. The lines of growth are minute. 



The hinge-processes are large, and project considerably, in common with others of the 

 eostatce which have much umbonal convexity. 



Dimensions of the larger of the specimens figured. 



Length measured upon the marginal carina . .36 lines. 



Across the valve at right angles to the carina . . 27 „ 



Breadth across the area . . . 9 „ 



Across the escutcheon of a single valve . . 3^ „ 



Thickness through a single valve . . 9 „ 



The diagnostic characters may be summarised as follows : 

 Considerable convexity of the valves. 

 Narrow elevated figure, and prominent umbones. 

 Anteal truncation. 



