TROPIDOLEPTUS CARINATUS IN CHEMUNG FAUNA 355 



have throughout its life history controlled its distribution. Recur- 

 rent faunas, therefore, afford special opportunities to discover the 

 factor most essential to the life of the fauna in a given case through 

 elimination of those factors which are common to the sediments 

 from which it is absent and in which it makes its earlier and later 

 appearances. With reference to the sediments, the recurrence 

 of a species after long absence from the section thus affords evi- 

 dence of similar conditions having been present in widely separated 

 formations, the importance and significance of which might other- 

 wise not have been apparent. 



In the light of these general considerations we may inquire 

 into the cause of the eastward retreat of the species which has been 

 shown to have occurred at the beginning of Genesee sedimentation 

 and its later westward and southwestward migration. The 

 evidence of such a movement has been cited on a preceding page. 

 The close of Hamilton sedimentation is marked in western New 

 York by a great change in the character of the sediments. The 

 sandy and often calcareous shales of the Hamilton are succeeded 

 by the thin band of the Tally limestone and the fissile black 

 carbonaceous shales of the Genesee in the central and western 

 parts of the state (see Fig. 1). When these black Genesee 

 and the succeeding lighter-colored shales of the Portage are 

 not entirely barren they are occupied by a fauna of "evident 

 deep-water habit having nothing in common with the pre- 

 ceding Hamilton fauna." 1 These black shale sediments follow- 

 ing the Hamilton extend southward beyond the Potomac River. 

 This sharp contrast between the sediments and faunas of the 

 Hamilton and those of the Genesee shales includes the total dis- 

 appearance of the large coral fauna of the Hamilton. The annihi- 

 lation at the close of the Hamilton of all fossils which, like corals, 

 require shallow waters, and the shifting of those species which 

 survived to the comparatively shallow coastwise waters points 

 plainly to the deepening of the sea at the close of Hamilton time. 

 Much additional evidence for the deep-sea conditions which pre- 

 vailed during Genesee and Portage time in western New York 



1 John M. Clarke, "The Naples Fauna in Western New York," N.Y. State Mus. 

 Mem. No. 6 (1904), 211. 



