370 T. E. SAVAGE ALEXANDRIAN SERIES IN MISSOURI AND ILLINOIS 



Atrypa marginal is (Dalman) Strophonella : sp. 



Atrypaputilla (Hall and Clarke) ~\Yhitfi el delta cf. cylindrica Hall 



Atrypa sp. Wh itfleldella sp. 



Camarotcechia near acin us Hall Bellerophon sp. 



Camarotcechia ? cliftonensis Foerste cf. Cyclora alt a 



Dalmanella elegant ula var. Conularia sp. 



Gypidula sp. Diaphoro stoma sp. 



Leptama rhomboidalis (Wilckens) Pleurotomaria sp. 



Loxoriema sp. 



Rhipidomella hybrida (Sowerby) Mytilarca cf. mytiliformis (Hall) 



Rhynchotreta lepida Savage Modiolopsis sp. 



Rhynchot ret a simpler Foerste Pterinea sp. 



Of the fossils of the Essex limestone, Atrypa putilla and Rhyncho- 

 treta thebesensis are common Edgewood species that are not known in the 

 higher Sexton Creek (Brassfield) limestone. Abundant also in the Essex 

 limestone are Camarotcechia ? cliftonensis. Rhynchotreta simplex, and 

 other species that have not been found in the Edge wood limestone of 

 Missouri and Illinois, but which occur in, or more probably immediately 

 below, a limestone containing Brassfield fossils in western Tennessee. 



The strata below number 1 of the section could not be seen at this 

 place, and 60 rods farther down the creek a thickness of 14 feet of Ma- 

 quoketa shale is exposed with no overlying limestone. 



The Essex limestone is thought to represent a distinct formation, al- 

 though its contact with the overlying Sexton Creek limestone is not 

 clearly exposed. Later studies may show that it should be regarded as a 

 member of the latter formation. It occurs below the Pentamerella f 

 manniensis horizon, which farther south in Illinois and Missouri marks 

 the basal layers of the Sexton Creek limestone, and are separated from 

 the Bowling Green member of the Edgewood limestone by a sedimentary 

 break. The Essex limestone is thus seen to be younger than the upper- 

 most Edgewood strata and older than the basal layers (containing Penta- 

 merella ? manniensis) of the Sexton Creek limestone of Missouri and 

 Illinois. 



THE SEXTOX CREEK (BRASSFIELD) LIMESTONE 



Occurrence and relation to other strata. — The name Sexton Creek 

 limestone was taken from Sexton Creek, in northwest Alexander County. 

 Illinois, along which stream these rocks are well developed and exposed. 

 These strata, which are considered the equivalent in time of some part 

 of the Brassfield east of the Cincinnati axis, overlap the Edgewood lime- 

 stone on the east and north in the Mississippi Eiver region and in the 

 south and east in northeast Illinois. They probably transgressed the 

 Edgewood strata over all the area of its distribution, but on account of 

 greater uplift along the west part of the Mississippi Eiver area in post- 



