126 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th See. 



known. Some of the echinoids found here were described by 

 Cotteau from the West Indies. The most widespread beds 

 recognized in the medial are correlated with the Bowden and 

 Gatun horizons and their fauna is an inshore facies of the 

 Bowden. Some of the echinoderms described by Cotteau from 

 the beds of supposed Eocene age are associated with character- 

 istic mollusks of the Bowden stage, thus showing that the age 

 of certain Antillean horizons are an inshore facies of the Bow- 

 den, upper Oligocene or lower Miocene. 



The senior writer is responsible for the identification of the 

 Pelecypoda, Gastropoda and some of the Echinoidea. The 

 junior writer determined most of the echinoids and the de- 

 scriptions of new species are written by him. 



Stratigraphy 



The geology of this region has recently been discussed by 

 Professor Dumble.^ The accompanying map illustrates the 

 general relations of the Tertiary beds north of Tampico. See 

 Plate 16, which was kindly supplied by Prof. Dumble. 



Professor Dumble has described this region as follows : 



Overlying the beds we have here referred to the Eocene, we find a 

 series of yellow sands, clays and calcareous beds which carry an Oligocene 

 fauna. We have called these the San Fernando from the fine exposures of 

 the beds in the vicinity of the town of that name on the Conchos River. 

 (Locality x4.) As will appear, these beds all belong to the Upper Oligo- 

 cene and up to this time no beds of the Lower Oligocene, like those of 

 the Buenavista River region with Orbitoidcs papyracca, etc., have been rec- 

 ognized in the area north of the Tamaulipas Range. From our present 

 knowledge it would seem that while the Lower Eocene deposits show a 

 gradual overlapping southward until the Conchos is reached, the Oligocene, 

 on the contrary, shows an overlapping northward to the same region, so 

 that along the Conchos the uppermost beds of the Oligocene are in con- 

 tact with the members of the Eocene there exposed. 



In the region of San Jose de las Rusias, which occupies the extreme 

 southern portion of this area, we have numerous exposures of the Upper 

 Ohgocene. (Localities x3, xS, x21, x22, and x23 are representative of 

 this region. R. E. D.) It apparently immediately overlies the Cretaceous 

 and is penetrated by eruptive rocks which are connected with or extend 

 eastward from the Tamaulipas Range. 



The lower beds of the Oligocene in this region are yellow clays, which 

 are altered in places and appear as hardened shales, and clayey limestones 

 carrying Cristellaria, Nummulites, corals and molluscan forms followed by 

 yellow sands and clays with an extensive fauna. The beds have a general 

 southeast dip. At San Rafael on the Zarizal River, at the extreme south- 



^Dumble, E. T. The Tertiary Deposits of Northeastern Mexico, Proc. Gal. Acad. 

 Set., 4th series, Vol. V, No. 6, pp. 163-193, 1915. 



