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 Burkart, 
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 has 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 palon- 
tologist in England, affirms that he could find no distinction 
whatever between the Indian Trigonie and those from Black- 
down*, Along with it Cardium Hillanum, Pecten quinquecostatus, 
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. Fé 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 Exocyre. 
Still more even than the Trigonie, we may regard the Exogyre 
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 (Hzo- 
gyra virgula, Knorri, 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. ui. p. 151. pl. 14. f. 3. 
