﻿54 BRITISH FOSSIL TRIGONI^. 



§ III. Undulate. 



Trigonia angulata, Sow. Plate XIV, figs. 5, 6. 



Tkigonia angulata, Sowerby. Mineral Conchology, 1826, pi. 508, fig. 1. 



— ANGULOSA, Agassis. Trigonies, 18-10, p. 9. 



— ANGULATA, lb. Trigonies, 1840, p. 50. 



— — Roemer, Versteinerungen, Oolith, 1836, p. 96. 

 , — — Morris. Catal., 1854, p. 228. 



— — D'Orhigny. Prodrome de Paleont., 1850, i, p. 308, No. 223. 



— — Lycett. Inf. Ool. Trigonias, Ann. and Mag. Nat. Hist., 1850, 



p. 427. 



Taking the figure in the ' Mineral Conchology ' as representing the typical form, 

 obtained from the Upper Ragstones of the Inferior Oolite at Nunney, near Frome, the 

 following will serve for its description : 



SheU sub-ovately elongated, convex ; nmbones obtuse, antero-mesial, much incur\'ed 

 and slightly recurved ; anterior side much produced and rounded with considerable 

 convexity ; lower border lengthened, nearly straight, but with a slight undulation or 

 excavation posteally ; hinge-border lengthened, concave, sloping towards the more 

 produced and narrow posteal extremity of the area, with which it forms a considerable 

 angle. Escutcheon moderately wide, depressed, and concave. Area narrow and flattened, 

 with two very small, but well-defined, bounding carinas, and in some specimens there is 

 an obscure line of minute tubercles bordering upon the median furrow, which is usually 

 distinct ; the bounding carinoe are also minutely tuberculated upon their upper portions, 

 they have always much curvature ; the area has transverse irregular plications, near to the 

 apices these become distinct, regular costellae. The sides of the valves have upon their 

 anteal portions a few narrow, inconspicuous, sub-tuberculated costse, which are directed 

 obliquely downwards to the middle of the valve, but not in the direction of the lines of 

 growth ; they form a curve or undulation, the convex border of which is directed towards 

 the marginal carina; for the most part their posteal extremities are bent suddenly 

 upwards or united to a larger nodulous series of costae or varices, which approach the 

 carina at a considerable angle, and the few lower costaj approach it almost perpendicularly ; 

 these are scarcely so numerous as the anteal series. The first-formed five or six costae 

 are without undulation, or are nearly concentric; all are slightly nodidous. The larger 

 example figured has some variability in its costae, which, however, does not entitle it to be 

 considered as a distinct variety. The few last-formed costas are irregular or confusedly 



