A NEW CRETACEOUS RUDISTID FROM THE SAN FELIPE 

 FORMATION OF MEXICO. 



By Timothy W. Stantox, 



Custodian of Mesozoic Invertebrate Fossils, United Stales National Museum. 



The geology of the Tampico region in eastern Mexico is so impor- 

 tant in its relation to the world's oil resources that any fact that will 

 aid in classifying the rocks of that region more accurately is welcome. 

 This fact as well as the interesting character of the fossil itself are 

 excellent reasons for describing a single characteristic fossil which 

 establishes the Cretaceous age of the San Felipe formation in its 

 typical exposures west of Tampico. This formation, which overlies 

 one of the chief oil-bearing horizons of the region, has yielded few 

 determinable fossils. By correlation with fossiliferous beds at 

 distant localities some geologists have referred it to the Cretaceous 

 while other have considered it Eocene. The species herein described 

 belongs to the family Radiolitidae, which, like the Hippuritidae, the 

 only other family of the Rudistae, is strictly confined to Cretaceous 

 rocks. 



The fossil was presented to the United States National Museum 

 by Mr. E. DeGolyer, chief geologist of the Compania Mexicana El 

 Aguila, S. A., who furnished the following statement concerning the 

 local geology : 



The San Felipe is a formation consisting of 600 to 800 feet of alternating thin-bedded 

 argillaceous limestone and shale known as the San Felipe beds. The limestones in 

 the San Felipe are hardly ever more than 8 inches to 2 feet thick and the interbedded 

 shales are quite similar. The limestones are much harder at the bottom of the section, 

 becoming more and more argillaceous toward the top until where last seen they are 

 really more bands of calcareous shale than limestone. The type locality lies between 

 the stations of El Abra and Valles in the State of San Luis Potosi. The San Felipe 

 beds are here exposed on the west flank of an asymmetrical anticline which brings up 

 the Tamasopo limestone and forms the topographically prominent Sierra del Abra. 

 The San Felipe is so called from a small ranch which lies 4 or 5 kilometers west of the 

 El Abra station. The fossil was collected on the north side of the track at a point 

 north 8° west magnetic from the fourteenth telephone pole [about \ mile] east of 

 Puente Diablo (Bridge of the Devil). The fossil was found in a hard blue limestone, 

 weathering gray, overlain by 3 to 4 inches of limy yellow clay, and it in turn overlain 

 by another thin limestone, all evidently belonging to the lower part of the San Felipe . 

 Stratigraphically, I should say it was found at 80 to 100 feet above the base of the San 

 Felipe. The formation where the fossil was collected strikes north 35° west magnetic 

 and shows a dip of 10° to the southwest. 



Proceedings U. S. National Museum, Vol. 59-No. 2379. 



453 



