458 PROCEEDINGS OF THE ACADEMY OF [1890. 



figures a number of fossils, some of which at least, seem to more 

 nearly represent the Middle Cretaceous than the Upper Cretaceous 

 system. Such is the Ammonite referred to Ammonites inflatus, which 

 it really appears to be. The second form of Ammonite, which is 

 identified with Ammonites planicostatus, is hardly that species, if the 

 drawing is at all to be relied upon ; the broad back and crowded 

 costse distinguish it from the well-known European species. It seems 

 to be more nearly a member of the group of A. Mantelli, if indeed, 

 it is not that species. The large gasteropod, which is identified with 

 Pterodonta inflata (Ceuomanian), is scarcely in a condition to be 

 determined ; it may, or may not, be that species, or it maybe a true 

 Tylostoma. Ramirez also figures (and comments upon its occurrence 

 here) a Posidonomya, but the imperfect drawing does not permit the 

 form to be recognized, or to be distinguished from a young Ino- 

 ceramus. 



I am not aware that the great limestone formation of South- 

 Central Mexico has been identified in the northern part of the 

 Republic east of the Sierra Mojada region, except along the im- 

 mediate borders of the Rio Grande. It is more than likely that it 

 outcrops in the State of Nuevo Leon, as Dr. White has evidence for 

 believing,^ but I know of no positive statement to that effect. The 

 supposed Cretaceous which Wislizenus describes as being found near 

 Mier, on the Alamo River, about four miles above its discharge into 

 the Rio Grande^ (State of Tamaulipas), is Tertiary, and with little 

 doubt either Miocene or Oligocene. The large oyster occurring 

 there is manifestly the species which is referred to by Penrose as 

 occurring on the Rio Grande, near Roma, Texas, and is doubtfully 

 identified with Conrad's Ostrea Georgiana:^ 



The oyster is, however, not Ostrea Georglana, but a much more 

 ponderous form, and one which is barely to be distinguished from 

 Ostrea Gingensis, of Schlotheim, a giant species of the Miocene de- 

 posits of continental Europe ; the specimen in my possession meas- 

 ures a foot in length and fully six inches in width. Dr. Penrose is, 

 I believe, justified in (doubtfully) referring the beds in which it oc- 

 curs to the base of the Grand Gulf Series ; the horizon represented 

 is surely above the Eocene. 



1 Am. Journ. Science, 1889, part 2, p. 441. 



2 Memoir of a Tour to Northern Mexico, U. S. Senate Publications, 1848, p. 



138. 



First Annual Report, Geo). Surv. Texas, 1889, pp. 46, 50, 56. 



