from the Inferior Oolite of the Cotteswolds. 259 



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 progress of growth that of the right valve de- 

 generates into one of the common oblique plications. The costse 

 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. 



Trigonia 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 with 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 Trigonia from the lower Oolite of Switzerland, 

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

 ticulata, 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 would have exhibited the peculiarities upon which I have 

 insisted as marking the species. 



Trigonia angulata t Sow. 



Trigonia angulata y Sow. Min. Con. tab. 508. fig. 1. 



Trigonia clavellata t Sow. Min. Con. tab. 87, the two lower figures. 



Shell elongated and rostrated, posterior border concave, ante- 

 rior border rounded ; umbones recurved ; area narrow, bounded 

 by small crenated carinae; costs narrow, closely arranged, 

 straight anteriorly, undulated posteriorly, where they form large 

 tuberculated varices, the few last varices directed downwards. 

 M. D'Orbigny (Prodrome de Paleontologie) considers Trigonia 

 undulata y Fromberg, to be only a synonym of T. angulata : in tin- 

 opinion I do not concur ; the figure of the two shells is essentially 



