CORRELATION OF STRATA 165 



statement of the case to say that these species are merely facies markers, 

 and that a limestone bed occurring at any level would be characterized 

 by them. Furthermore, the Encrinal limestone in both regions marks 

 the period when the corals Heliophyllum halli and Cystiphyllum vesiculosum, 

 as well as the Favositids, made their first appearance. These, in the 

 Thedford region, are most abundant in the coral layer just above the 

 limestone, while in western New York the Favositids characterize the 

 Encrinal limestone and the Cyathophylloid corals the coral layer above it . 



Sixty miles east of Eighteen-mile creek, in the Genesee valley, the 

 Encrinal limestone has the same lithic and faunal characters, and the 

 characteristic corals occur 5 feet above it.* One hundred and twenty feet 

 higher up in the same section is another calcareous layer, but poor in 

 fossils, and a short distance above this corals are again common. Ten 

 feet higher another limestone bed with corals occurs. 



Although we have in the Genesee section a repetition of limestone 

 beds and coral zones, we believe that the lowest of these is the equivalent 

 of the Encrinal of western New York, for it is the only one which agrees 

 with it faunally. We furthermore believe that the bed designated as the 

 Encrinal limestone in the Thedford region is the stratigraphic equivalent 

 of the bed known in New York by the same name. We believe that the 

 facts warrant the assumption that the bed thus designated could, if ex- 

 posures were sufficiently numerous, be traced with little or no interrup- 

 tion from the Genesee valley to Thedford and beyond. 



If we accept the stratigraphic equivalency of the bed in question we are 

 enabled to make comparisons between the lower and upper beds of the 

 Thedford and Eighteen-mile Creek regions. As far as we can judge from 

 our collections, the lower beds of Eighteen-mile creek are richer in species 

 than the corresponding ones at Thedford. In both cases, however, the 

 characteristic Spirifer is a variety of S. mucronatus, which has not departed 

 far from the primitive mucronate type which was the radicle of the group 

 of varieties classed under this name. In the Eighteen-mile Creek region 

 the mucronate type occurs, though chiefly in the lower beds. A modi- 

 fied form, which resulted through an increase in height, with but a mod- 

 erate increase in width, still retaining the characteristic plicated sinus 

 and impressed fold, takes its place in the upper beds. In the Thedford 

 region, on the other hand, the primitive characters were retained longer, 

 thus producing a wide, much plicated fprm, as discussed below. This 

 species retains the mucronate character until it is near the adult stage, 

 when the characteristic change in growth appears. We might from this 

 perhaps expect to find the normal form of S. mucronatus, which never 



* Clarke and Luther : Livonia salt shaft. 

 XXV— Bull. Geol. Soc. Am.. Vol. 13, 1901 



