﻿158 BRITISH FOSSIL TRIGONIiE. 



Lyriodon costatum, Bronii. Letha;a Geognostica, tab. xx, fig. 4, 1837, 1838. 

 Teigonia scclpta, Lycett. Handbook Cotteswold Hills, p. 65, 1857. 



— — Sharp. Oolites of Nortliamptonshire, Quart. Journ. Geol. Sec, 



vol. xxi.v, p. 293, 1873. 



— EoLANDi (Lyc), Cross. Geol. of N. W. Lincolnshire, Quai-t. Journ. Geol. 



Soc, vol. xxxi, p. 125, 18/5. (Var. of 

 T. sculpt a). 



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



Shell siibovate or ovately oblong, moderately convex ; iirabones prominent, pointed, 

 snbantcrior, 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 carinas ; the intercarinal 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 costse, about twenty-seven in fully developed forms, 

 are curved obliquely or subconcentric, are somewhat narrow and flattened, with 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-fonned 

 costse 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 urabones 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 sui'face-ornaments, together with 

 those of the bounding carinae, 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. cosfata. 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 Localiiies. 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 



