166 SHIMER AND GRABAU — HAMILTON GROUP OF THEDFORD 



passes beyond the mucronate stage, in the lowest beds of the Thedford 

 region, but unfortunately these are not exposed in this locality. 



Whether or not any portion of these lower beds carries the character- 

 istic Marcellus fauna of New York is, of course, not known. Logan 

 (page 385) mentions a bed of black shales holding Stropheodonta inequi- 

 striata, Airypa reticularis, and Chonetes in the bottom of a ravine at Aus- 

 tin's mill. These beds he states to be 50 or 60 feet below the Encrinal 

 limestone, and suggests that they " may possibly indicate the passage 

 which occurs in New York between the Marcellus and Hamilton shales." 



The abrupt cessation of the long-winged Spirifer mucronatus and the 

 equally abrupt appearance of the short-winged variety suggests that the 

 former became extinct or was driven out from this region, while the 

 latter immigrated from some other locality, where it had developed. In 

 this connection the occurrence of a few specimens of the variety arko- 

 nensis in the Encrinal limestone of western New York is interesting.* 



So far as has yet been determined, Spirifer- scidptilis is confined to the 

 Encrinal limestone of western New York and Canada, though in the 

 Genesee valley it occurs above the Encrinal limestone. 



The order of appearance of the diagnostic Spirifers is, so far as is at 

 present known, as follows : 



Thedford Region. Western New York Region. 



1. ? S. mucronatus (radicle)? V. S. mucronatus (radicle) 



(mucronate) not found. (mucronate). 



2. S. mucronatus arkonensis 2 7 . S. mucronatus (short- winged, high, 



(long-winged, non-mucronate). non-mucronate). 



3. S. scidptilis. of / S. sculptilis. 



c a. S. mucronatus thedfordensis. { S. mucronatus arkonensis. 



' \b. S. consobrinus. 4'. S. consobrinus. 



5'. S. tullius. 



The occurrence of S. consobrinus in the Thedford fauna is noted by 

 Schuchert, who refers it to the " middle third." This undoubtedly is 

 the coral layer. 



As far as our study of the Thedford fauna throws any light on the ques- 

 tion of the direction of migration of the Hamilton faunas within the 

 intercontinental Hamilton sea, a general eastward migration seems to be 

 indicated. There appear to be exceptions to this, however. One of 

 these is S. mucronatus thedfordensis, which is most naturally derived from 

 the short-winged and high variety of S. mucronatus, characterizing the 

 upper Lower Hamilton of western New York. Again, Tropidoleptus cari- 

 natus is represented in our collections by a single specimen from the En- 



* Grabau, 1898. 



