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BRITISH FOSSIL TRIGONLE. 



formation, more especially by the two more abundant of its forms, T. gibbosa and T. 

 Damoniana, I prefer to regard (provisionally at least) the two allied forms from Boulogne 

 and Devizes as constituting only well-defined varieties of one species. 



Dimensions. — Length 16 lines, height 13 lines, thickness through a single valve 

 4 lines. 



Trigonia excentrica, Park. Plate XX, figs. 5, 6 ; Plate XXI, figs. 6, 7 ; Plate XXII, 



figs. 5, 5 a. 



Trigonia excentrica, Parkinson. Org. Rem., vol. iii, pi. xii, 1811. 



— sin u ata, lb. Ibid., fig. 13. 



— excentrica, Sow. Mm. Conch., vol. iii, p. 11, tab. 208, figs. 1, 2, 1821. 



— affinis, Miller and Sow. Ibid., tab. 253, fig. 3, 1821. 



— — Befrance. Diet, des Scien.Nat., tab. lv, p. 297, 1828. 



— — Pusch. Polens, Paleontologie, p. 61, 183/. 



— excentrica, lb. Ibid. 



— — Agassiz. Trigonies, p. 9, 1840. 



— affinis, lb. Ibid., pp. 9 et 52, 1840. 



— excentrica, D'Orbigny. Prodrome de Paleont., 1850, vol. ii, p. 160, 



No. 328. 



— stnuata, lb. Pal. Franc., Terr. Cret., torn, iii, p. 147, pi. 293, 1843. 



— — lb. Prodrome de Paleont., vol. ii, p. 161, No. 323, 1850. 



— — Morris. Catalogue, p. 229, 1854. 



— excentrica. Ib. Ibid., p. 228, 1854. 



Shell inequilateral, subovate, rather depressed and thin in the very young condition, 

 becoming thick, with a considerable convexity, in an advanced stage of growth ; umbones 

 pointed, erect, little produced, situated about two fifths the length of the valve from the 

 anterior border. Anterior side produced, its border curved elliptically with the lower 

 border ; hinge-border nearly straight, or in some examples slightly concave, sloping 

 obliquely downwards, lengthened, terminating in a posteal extremity, which is rounded 

 but attenuated. Area narrow, slightly concave near to the umbo, where the valve forms 

 an oblique angle, separating the area from the anteal portion ; the angularity soon 

 disappears, the area then acquires some convexity, and has no distinct separation from 

 the other portion of the surface excepting that a space anteal to the area becomes 

 somewhat depressed near to the lower or pallial border. The other portion of the shell 

 is covered by a series of very numerous, inconspicuous, slightly elevated, longitudinal or 

 horizontal costse, which are indented anteally by oblique intersecting lines of growth ; the 

 costse are regular and distinct, crossing the entire valve near to the umbo, but they soon 

 disappear over the posteal third of the surface, and examples of adult growth have the 



