386 M. Von Buch on the characteristic Fossils 



Degenhardt's 'American Fossils ' (Berlin, 1839), and as still more 

 clearly appears from Alcide D'Orbigny^s learned work on Bous- 

 singault^s collections, it follows that the strata discovered by 

 Galeotti above Tehuacan must be joined, with all their organic 

 remains, to the middle chalk. The collections made by Bm-kart, 

 Councillor of Mines in Bonn, in the mountains of Guanaxuato, 

 contain nothing opposed to this view. We do not again find this 

 Trigonia further south in America, in Peru or in Chili, at least 

 it has not yet been observed in the cretaceous strata so common 

 in all this region. On the other hand it appears in other quarters 

 of the globe. The enterprising and talented Director Kraus of 

 Stuttgart has brought from Zwartkopp, Algoa Bay at the Cape 

 of Good Hope, a Trigonia which in all essential characters agrees 

 with the Trigonia aliformis — even the acute angle, under 60°, of 

 the anterior and posterior margins, and the direction of the ribs 

 with the fine crenulations on them, Herr Kraus iias named this 

 shell Trigonia ventricosa. It is almost surprising to find this same 

 Trigonia aliformis in the chalk hills which appear as if blown by 

 the winds over the vast peninsula of Hindostan, quite in the 

 south, near the point at Verdachellum to the south-west of Pon- 

 dichery, and nearly in the same circumstances as in Europe and 

 America. Prof. Edward Forbes, the most distinguished palaeon- 

 tologist in England, affirms that he could find no distinction 

 whatever between the Indian TrigonicB and those from Black- 

 down*. Along with it Cardium Hillanum, P eaten quinquecosiotus, 

 orbicularis, obliquus, occur, so that Mr. Forbes has no difficulty 

 in referring the strata of the hill of Verdachellum to the upper 

 greensand and the gault, or exactly the place to which the thick 

 beds of St. Fe de Bogota, of Tehuacan and of Alabama, must be 

 referred. Still the Indian beds contain a great number of forms 

 which are peculiar to them alone, and perhaps bear some rela- 

 tion to tropical conditions of climate, and which by themselves 

 might be a reason for suspecting that these hills form a very 

 highly developed tertiary formation. The Trigonia alone is suf- 

 ficient to lead us to a better conclusion. It is a characteristic 

 fossil. 



3. The Exogyr^. 



Still more even than the Trigonia, we inay regard the ExogyrcB 

 as a stamp impressed on the whole cretaceous formation. These 

 singular oysters appear for the first time in the Jura deposits, 

 but only small, hardly an inch in size, and in most cases {Exo- 

 gyra virgula, Knorrii, spiralis, auriformis) scarcely larger than 

 beans. Wherever they are several inches in size, it may be un- 

 conditionally assumed that they declare the formation to be cre- 



* Geol. Trans. 2nd ser. vol. vii. P. iii. p. 151. pi. 14. f. 3. 



