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BRITISH FOSSIL TRIGONE. 



Lyriodon costatum, Bronn. Lethaea Geognostica, tab. xx, fig. 4, 1837, 1838. 

 Tbigonia sculpta, Lycett. Handbook Cotteswold Hills, p. 65, 1857. 



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



vol. xxix, p. 293, 1873. 



— Eolandi (Lye), Cross. Geol. of N. W. Lincolnshire, Quart. Journ. Geol. 



Soc., vol. xxxi, p. 125, 1875. (Var. of 

 T. sculpta). 



— sculpta, Judd. Mem. Geol. Survey, Rutland, &c, p. 281, 1875. 



Shell subovate or ovately oblong, moderately convex ; umbones prominent, pointed, 

 subanterior, and slightly recurved ; anterior side short, its border curved elliptically with 

 the lower border ; superior border straight, lengthened, forming an obtuse angle with the 

 siphonal, the length of which it exceeds by one fourth. The escutcheon is lengthened, 

 flattened, and depressed ; it has some oblique irregular plications which take the direction 

 of the lines of growth. The area has some convexity, more especially in the right valve ; 

 its greatest breadth is somewhat less than one third the breadth of the entire valve ; it is 

 rendered conspicuously bipartite by the considerable depression of the superior half ; it is 

 bounded by two deeply dentated carina? ; the intercarpal costellse are few, large, and 

 somewhat irregular ; all are coarsely denticulated and in some specimens the first costella 

 of the lower or outer half is slightly larger than the others, forming a median carina, a 

 feature which is not distinct in the right valve, which has the lower half of the area more 

 elevated and its costella? larger. The marginal carina is large in both the valves and its 

 denticulations are very prominent. The costa?, about twenty-seven in fully developed forms, 

 are curved obliquely or subconcentric, are somewhat narrow and flattened, witli little 

 elevation ; anteriorly their extremities are simply curved upwards ; their posteal extremities 

 approach the marginal carina nearly at right angles. In the right valve the few last-formed 

 costa? have frequently some irregularity and less prominence, or become imperfect. 



The foregoing description applies to the larger or typical form, a species as large as 

 T. costata, from which it differs in some important features. The general figure is less 

 trigonal ; it has less convexity at the angle of the valve ; the umbones are more pointed 

 and terminal ; the anterior border, although little produced, has nothing of the truncation 

 of the other species ; the area is somewhat less wide ; its surface-ornaments, together with 

 those of the bounding carina?, are much larger or more coarsely sculptured ; the costae are 

 curved obliquely, having a simple curvature upwards towards the anterior border ; they are 

 therefore destitute of the anteal undulation and slight double flexure which characterise 

 those features of T. costata. The test is thick and the hinge-processes are so large that 

 they occupy nearly one third of the interior of the shell. 



Positions and Localities. T. sculpta has occurred rarely in the highest or Ammonite- 

 bed of the Supra-liassic Sands at Haresfield Hill, near Gloucester ; its more common 

 position is the Gryphite-grit or Lower Trigonia-grit of the Cotteswold Hills, near 

 Stroud and Cheltenham, where it has occurred abundantly ; other localities are Dundry 



