DEVONIAN. 309 



Later work has not sustained the identification of Ithaca with Chemung. 

 Wilhams ^^^ first showed that Ithaca and Portage are equivalent. He said : 



Although the faunas [of the Ithaca and Chemung] are very similar, there can be no doubt 

 that along the present meridian they represent two distinct geological stages. That they 

 blended in some measure farther east may be possible, in the same way that it is probable 

 that on going westward the Marcellus and Genesee stages blended. 



Although I do not know that the Ithaca fauna is an early stage of the Chemung fauna, 

 I am persuaded that the two may be readily distinguished by their fossils. 



That the typical Chemung fauna is thus distinct from that of the Ithaca group and char- 

 acteristic of a later stage, is shown, paleontologically, by the following considerations: 



The genera Spirifera, Orthis, Strophodonta, and Productella are common to both faimas, 

 and are represented by numerous individuals at almost any fossiliferous exposures of either 

 group. But for each genus the species are different. Spirifera is represented in the Ithaca 

 group by Sp. mesocostalis var. acuminata and the first variety of Sp. mesostrialis. 



The Chemung group is characterized by Sp. disjuncta, Sp. mesocostalis 2d var., a large, 

 coarsely plicated, broad form, and Sp. mesostrialis 2d var., the wide mucronate form, neither 

 of which is seen in the Ithaca group. 



Orthis, in the Ithaca group, is 0. impressa of the narrow variety, rarely wider than long. 



In the Chemung group 0. impressa is the second variety; wide form, with broad sinus; 

 also there are 0. tioga and 0. carinata, neither of which is known in the Ithaca group. 



The Strophodontas of the Ithaca group are Str. mucronata, and the closely allied variety 

 of Str. perplana var. nervosa. 



In the Chemung group Str. cayuta is the prevailing form, and a coarser, more irregulai 

 form of Str. perplana var. nervosa. 



Str. demissa is reported from both groups but is extremely rare in either. 



Productella is represented in the Ithaca group by P. speciosa and a small form I have 

 identified as P speciosa, small variety. In the Chemung it is P. lacJirymosa and P costatula. 



Besides these genera, Streptorhynchus is common in the Chemung group, and it is extremely 

 rare, if it appear at all, in the Ithaca group. i 



Amlocmlia umbonata var. gregaria is abundant in some stages of the Chemung group but is 

 rarely ever seen at Ithaca. The latter two forms are seen below the Ithaca group, hence then- 

 absence there is evidence of modified fauna rather than extmction. 



These differences in the prevailing varietal or specific characters of common genera, which 

 (as genera) are known to be common for a considerable range below and above the stages 

 under consideration, I take to be more reliable evidence of actual difference in horizon than 

 would be any number of distinct species of different genera in the two faunas. 



In the study of the Devonian of New York to which the bulletin just cited was 

 a first contribution, Williams has developed essential general principles regarding 

 the geographic and chronologic relations of faunas. The principles and the dis- 

 cussion of the evidence do not pertain to this account of stratigraphy, but the 

 application to the relations in New York is appropriate. Under the heading 

 "Faunal dissection of the Devonian," Williams"^* expresses the results of special 

 studies as follows : 



Previous work in correlation had been conducted on the fundamental assumption that 

 identity of fossils is sufficient evidence of identity of the formations containing them. In other 

 words, it had been assumed that for purposes of classification in the time scale the formation 

 and the fauna are identical. The way in which fossils have been customarily labeled has pre- 

 vented a testing of the truth of this assumption. If there be any distinction in time value 

 between the formation and its faima, it is difficult to demonstrate it so long as the only name 

 and designation of the fauna is that of the formation in which it was originally found. 



