448 PROCEEDINGS OF THE ACADEMY OF [1890- 



on the earth's surface which in an equal area abounds so largely in 

 fossil remains (p. 220). The abundance of fragments of large 

 Ammonites, supposed to measure from 50 to 60 centimetres in 

 diameter, is especially commented upon. 



With due deference to the learning and experience of the authors, 

 I am constrained to believe that the formation in question is'tiotof 

 Jurassic age, but Cretaceous, and I may add, late Cretaceous. It is 

 true, I have not been in the precise region described by Nyst and 

 Galeotti, but my observations extend around and about it in vari- 

 ous directions, and I have nowhere in the limestone region found 

 any good indications of Jurassic fossils. Cretaceous forms, on the 

 other hand, are very abundant, and just in the mountains of Tehua- 

 can do they appear in great numbers. Furthermore, the fossils of 

 Tehuacan, from the region of the large quarries of Mexican onyx, 

 are almost precisely identical with those (Hippurites, Radiolites, 

 etc.) which I have seen in the limestones of Coatepec (Jalapa), of 

 Atoyac, the Cerro de Escamela, near the town of Orizaba, of Apasco, 

 Yautepec, Coalcoman (in Michoacan), etc., showing the broad ex- 

 tent of an identical formation. Nyst and Galeotti in the memoir 

 above cited describe twelve species of fossils — Trigonia plicato- 

 costata, Ostrea acuticosta, 0. similis, Cerithhwi suturosum, C. Busta- 

 mentii, C. cingulatum, Terebra minnta, Ammonites Rioii, A. re- 

 conditus, Cidarites propinquue, C. pustidosus, and C. glandiferus — 

 and it is upon the supposed relationships of these that the reference 

 to the Jurassic formation is made. Cidarites propinquus (Miinster) 

 and C. glandiferus (Goldfuss) are considered to be well-known species 

 from the Jurassic deposits of Germany. The figures illustrating the 

 above are apparently faithfully drawn, but I fail to find anything in 

 them which would indicate that they deal with Jurassic, rather 

 than with Cretaceous, fossils. Trigonia pi icato-costata, which appears 

 to be one of the most abundant of the fossils occurring in the region 

 under consideration, represents a well-recognized Upper Cretaceous 

 type of Trigonia, that of T. alcpformis and T. scahricola, and is far 

 removed from the distinctive Jurassic forms. Indeed, Nyst and 

 Galeotti themselves recognize the relationship with the first-named, 

 from which they distinguish it by the character of the costal orna- 

 mentation. While I am not certain that the Mexican species is 

 really distinct, yet, in the absence of specimens for direct compar- 

 ison, it may be well to consider it so. But I believe there can be no 

 doubt as to the horizon which it represents. D'Orbigny, in his 



