﻿912 



BRITISH FOSSIL TRIGONItE. 



excepting at the boundary of the escutcheon, where they form a carinal angularity ; the 

 escutcheon is well defined, and its surface equally, as in the area, is closely and 

 ])rofusely tuberculated. The ligamental cavity is larger and more lengthened than in 

 the other form. Usually the rows of carinal nodes cannot be distinguished over the 

 middle and posteal thirds of the valve, the entire surface of which is occupied by the 

 large, crowded tubercles. This appears to be the only Belgian variety of T. dadalea ; 

 the convexity of the valves is greater than in the typical form, resulting from the greater 

 breadth of the escutcheon. 



With the foregoing species is associated another abundant and equally characteristic 

 form, which in the Belgian beds seems to replace the aliformis group of Blackdown ; 

 this is the Trigonia EliscB of Cornet and Briart. I am indebted to the liberality of 

 Dr. Charles Barrois, of the Faculty of Sciences, Lille, for the gift of a series of each of 

 these Trigonice. He refers them to the zone of Am, injlatus, Sow. T. EUscb is a 



Trigonia Elisa, Cornet and Briart. 



much ornamented and characteristic example of the Scabne, allied to the aliformis 

 group, and in common with others of its allies remarkable for the great length of the 

 hinge-border and the shortness of the siphonal border ; it is moderately convex anteally, 

 produced and attenuated posteally. The rows of costae covering the sides of the valves 

 are very numerous and closely arranged, with rounded, depressed nodes ; they are 

 concentrically or obliquely arranged anteally ; the rows rapidly diminish in size posteally, 

 where their ornamentation becomes obscure. All the costae have their posteal portions 

 much attenuated, straight, perpendicular, or inclined retrally. The escutcheon is narrow 

 and concave ; it has delicate, closely placed, transverse costellee, which also pass across 

 the upper half of the narrow and flattened area, the lower half of which is smooth ; 

 there are no distinct carinal elevations, but the boundaries of the area and escutcheon 

 are well defined. 



Trigonia? modesta, Tate. Plate XLI, figs. 13, 13 a. 



TuiGONiA MODESTA, Ta/e and Blake. 'The Yorkshire Lias,' LameUihraHchiata, by 

 R. Tate, p. 386, pi. xiv, fig. 4, 1876. 



