328 CORRELATION OF THE UPPER CRETACEOUS FORMATIONS 



direct faunal evidence for correlating it with the Magothy of Maryland, 

 with which, however, it is probably in part synchronous. The correlation 

 of the Austin chalk with the Mortoniceras subzone has already been sug- 

 gested by Stanton 1 and by Stephenson. 2 



The Texas fauna is remarkable in that it exhibits very clearly the 

 southern fades, as typified by Radiolites, which characterizes the Upper 

 Cretaceous deposits of both the eastern and western hemispheres. It was 

 furthermore laid clown in much deeper water than the Mortoniceras 

 subzone of Maryland or of the greater part of the Gulf. In spite of the 

 differences in facies and the very imperfect knowledge of the Texas 

 Cretaceous faunas, four, or possibly five, of the twelve or thirteen species 

 which in the South Atlantic states and Gulf are restricted to the Mortoni- 

 ceras subzone have been identified from the upper part of the Austin chalk. 

 These are: Ostrea diluviana and the four ammonites, Placenticeras 

 planum, Placenticeras gaudalopce, Mortoniceras aff. texanum, and Bacu- 

 lites ( ?) anceps. Four additional species from the small Austin fauna are 

 restricted to the Exogyra ponderosa zone (sensu lato), namely, Gryphwa 

 aucella, Exogyra ponderosa, Radiolites austinensis and Baculites asper. 

 In short, four out of the six species of ammonites which have been deter- 

 mined from the Mortoniceras subzone of the Gulf occur in the Austin 

 chalk, while one of the two remaining is represented by a very closely allied 

 form. The affinities to the ammonites of the Mortoniceras subzone of 

 Maryland are only a little less obvious. Baculites asper is present in both, 

 while Mortoniceras delawarensis has a close analogue in Mortoniceras 

 texanum, and Placenticeras placenta in Placenticeras planum and P. 

 guadalopce. On the other hand, Placenticeras placenta is recorded by 

 Meek from the Colorado group of the West, and Stanton states that still 

 more typical forms of this species occur in association with Buchiceras 

 swallovi in the Inoceramus labiatus zone (Eagle Ford formation) of 

 Texas. 



The only critical species which has been determined from the Taylor 

 marls, which overlie the Austin, is Exogyra ponderosa. The correlation 



1 Stanton, T. W., 1909, Jour. Geol., vol, xvii, p. 419. 



2 Stephenson, L. "W., 1914, U. S. Geol. Survey, Prof. Paper 81, p. 32. 



