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BRITISH FOSSIL TRIGONLE. 



Trigonia HEMISPH.ERICA, Lyc. Plate XXXI, figs. 4, 5, 6, 7, 8. Var. Gregaria, 

 Plate XXXIII, figs. 4, 5, 6. 



Trigonia hemispheric.*, Lycett. Ann. and Mag. Hist., vol. xii, tab. xi, fig. 2, young 

 specimen, 1850. 



— — Morris. Catalogue, p. 228, 1854. 



— — Lycett. Cotteswold Hills, Handbook, p. 137, pi. iv, fig. 4, 



1857. 



— — Phillips. Geology of Oxfordshire, p. 162, 1871. 



— — Sharp. Oolites of Northamptonshire, Quart. Journ. Geol. 



Soc, pp. 267, 293, 1873. 

 — ■ — Cross. Geology of North-west Lincolnshire, Quart. Journ. 



Geol. Soc, pp. 121, 124, 1875. 



— — (drawf variety). Ibid., pp. 121, 125. 



— — Judd. Mem. Geol. Sur. Rutland, &c, p. 153, 157, 281, 



1875. 



Shell ovately trigonal, convex, umbones prominent, pointed, slightly recurved ; 

 anterior side short, its border curved elliptically with the lower border, posterior side 

 moderately lengthened. Escutcheon depressed, its superior border somewhat raised, 

 moderately wide, obliquely closely plicated, its length greater than that of the siphonal 

 border of the area with which it forms an obtuse angle. The area is of considerable 

 breadth, or equal to one third of the entire valve, it is for the most part flattened, is 

 depressed L and slightly concave ; the right valve has the lower half or that adjoining the 

 marginal carina raised or convex. The marginal carina is large, with elevated, deeply 

 indented irregular plications, these are in some instances scarcely more numerous than 

 the costae ; the inner carina is also conspicuous from its prominent close set row of 

 obtuse nodes. There is no median carina in the right valve ; the left valve has occasion- 

 ally some approximation to a median carina in the greater elevation of one of its costellae. 

 The costellae of both valves are prominent and closely arranged with large, deeply 

 indented reticulations ; the lower half of the area has eight or nine, the upper half six or 

 seven costellae. In the very young state, the right valve has the costellae of the lower 

 half of the area fewer and larger, which is a feature not unusual with other species of the 

 Costata. 



The other portion of the surface has a very numerous regular series of narrow, 

 elevated, closely arranged, rounded, or sometimes subacute costae, varying from 40 to 55 

 or even exceeding the latter number, occasionally in specimens measuring two inches or 

 upwards upon the marginal carina ; those of the left valve are slightly enlarged at their 

 posteal extremities and terminate abruptly at the narrow deep antecarinal groove; 

 anteally their extremities become attenuated, and have a single curvature upwards to the 



