from the Inferior Oolite of the Cotteswolds. 237 



Var. 4. sculpta. 



In dimensions this well-marked variety equals the typical form ; 

 in certain localities it occurs in the Gn^hite grit in immense 

 abundance ; it is distinguished from the tj^ical form by several 

 conspicuous characters ; the figure is less trigonal, the anterior 

 border being destitute of any truncation ; the umbones have less 

 prominence and are less recurved, the area is larger, flatter, it is 

 less concave, and occupies a much larger proportion of the sur- 

 face of tlie shell ; the marginal and inner carinse are larger, less 

 cuned, and in common with the intercarinal plications, they are 

 much more strongly dentated ; o^ing to this prominence of the 

 plications the median carina is much less conspicuous, the pos- 

 terior half of the area is more depressed than the anterior, so 

 that a distinct mesial division is formed irrespective of the me- 

 dian carina. In the young state the median carina is distinct in 

 each valve, but in progi-ess of growth that of the right valve de- 

 generates into one of the common oblique plications. The costae 

 are large and elevated, but they have not the graceful double 

 curvature of the typical form. 



This variety therefore differs from the typical form in its pro- 

 portions, its general outline, and in the greater prominence of 

 its surface ornaments ; but the peculiarity which distinguishes 

 the species in the character of its area is present in all the va- 

 rieties, and serves to separate them from all of the allied costated 

 forms. 



Ti-igonia costata is stated to occur over the whole of Europe, 

 and there is even a presumed variety of it from Cutch, figured 

 and described by Mr. James Sowerby in the ' Geol. Trans.' vol. v. 

 2nd Ser. ; it is, however, not improbable that a further acquaint- 

 ance ^dth the varieties of this shell and of allied costated species 

 may lead eventually to altered views, both of their stratigraphical 

 and geographical distribution. 



The costated Trigoni(s from the lower Oolite of Switzerland, 

 figured by Agassiz under the names of T. lineolata and T. den- 

 ticuhta, would appear from his figures and descriptions to be 

 distinct from T. costata ; the accuracy of the figures in the ' Pe- 

 trefacta ' of Goldfuss is exemplified in the fidelity with which the 

 artist has delineated the area of the right valve in the young spe- 

 cimen, although its peculiarities are not alluded to in the de- 

 scription ; the typical figure of Agassiz is correct, but it may as 

 confidently be asserted, that the figure of the right area in the 

 same plate has incautiously been transferred from the left valve, 

 or it woidd have exhibited the peculiarities upon which I have 

 insisted as marking the species. 



