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BRITISH FOSSIL TRIGONLE. 



to the same species ; they are without the crenulations upon the pallial border which 

 characterise both T. aliformis and T. Vediana. 



Trigonia Fittoni, Desk. Plate XXIII, figs. 4, 4 a, 4 b, 5. 



Trigonia Fittoni, Beshayes. Leymerie, Mem. de la Soc. geol. Fr., torn, v, pi. ix, 



fig. 6, 1842. 



— — B'Orbigny. Paleont. Fran., Terr. Cret., vol. iii, p. 140, pi. 290, 



figs. 1—5, 1843. 



— — lb. Prodrome de Paleont., vol. ii, p. 137, No. 243, 1850. 



— — Cotteau. Moll. foss. de l'Yonne, p. 77, 1854. 



— — Raulin et Leymerie. Statistique de l'Yonne, p. 4/3, 1858. 



Shell ovately oblong, very convex anteally, depressed and somewhat attenuated 

 posteally ; umbones large, much elevated, slightly recurved, much incurved, their apices 

 are in contact when the valves are closed, antero-mesial ; anterior side short, its border 

 curved elliptically with the lower border ; superior border concave ; siphonal border 

 short, nearly perpendicular, rounded at its extremities. Escutcheon small and 

 lengthened, moderately wide and concave anteally, narrow and pointed posteally; its 

 superior border is much raised upon the anteal half of its length bounding the ligamental 

 fissure, which is lengthened, extending both anteally and posteally to the umbonal 

 apices. Area moderately wide, destitute of bounding carinas, traversed by a deeply 

 indented longitudinal mesial furrow ; its surface, in common with the escutcheon, has a 

 numerous (about 20) series of delicate, narrow, regularly papillated costellae, which 

 descend almost perpendicularly from the superior border ; they enlarge slightly as they 

 cross the area, and again curving upwards outwardly meet the extremities of the pallial 

 costae at the divisional line of the valve, forming with them acute angles ; about three 

 fifths of the length of the divisional line is occupied by the extremities of these costellae, 

 which extend upon the superior half of the area even to its posteal extremity ; the posteal 

 portion of the lower moiety of the area," therefore, is traversed only by delicate lines of 

 growth. The other portion of the surface is occupied by about twenty rows of elevated, 

 rounded, concentric, regularly papillated costae, attenuated at both of their extremities ; 

 the last-formed seven rows curve upwards almost perpendicularly to the angle of the 

 valve, their lower extremities forming a jagged outline at the lower border, alternating 

 with the projecting extremities of the costae upon the other valve. The anterior face of 

 the shell has the wide intercostal spaces occupied by numerous small, short, horizontal, 

 rather obscure, supplementary costellae, two or three of which occur in each space, a 

 feature also seen in T. aliformis ; each of the papillae or little nodes fringing the costae 

 has a little pillar or plication passing downwards, a feature which is distinct only 



