MARCELLUS LIMESTONES AND THEIR FAUNAS 137 



characteristic species (Orthotlietes be 1 lulus, Cho- 

 netes cf. yandellanus, Ambocoelia praeum- 

 b o n a) a single well defined example ofAgoniatites ex- 

 pa n e u s was obtained. It constitutes the first manifestation 

 in New York state sections of the Hamilton fauna iii' its pre- 

 nuncial invasion from the west, when it was complicated with 

 the fauna already on the ground and failed to gain a lasting foot- 

 hold or to develop favorably. The Agoniatites expansus 

 occurring here may, in view of the other evidence, properly be 

 regarded, like the species last mentioned, as a survivor of the 

 Onondaga fauna, 



CONCLUSION 



The Agoniatite limestone fauna was an invader from the toest, 

 dating from the closing phase of the Onondaga limestone. Directly 

 in its train followed the, prenuncial cohat^ts of the Hamilton fauna. 

 The former held the footing it had gained while the latter yielded to 

 unfavorable conditions und temporarily retired from the field. 

 w 



Flint creeK Monlius Chern/ 



Valley 



STAFFORD Limestone :2n(l Hamiiron invasion 

 AGONIATITES and 1st HamiUon invasion 



AGONIATITES Limestone 



Onondaga limestone ^~^~^i . i-S-S-h^- 



Fig. 2 Diagrammatic representation of the relation of the Hamilton fauna! invasions 

 to the sediments of Onondaga and Marcellus time. 



The facts recorded above indicate quite clearly the descent of 

 the Agoniatite horizon westward from an elevation of fully 50 

 feet above the summit of the Onondaga limestone to and 

 probably into that limestone itself, a condition which stra- 

 tigraphy interprets as a replacement in central and eastern New 

 York sections, of the upper layers of the latter by the lower beds 

 of the Marcellus shales or, conversely, the occupancy of the field 

 in western New York by the Onondaga sea after Marcellus 

 deposition had for some time been under way in central and east- 

 em New York. The fauna attained its highest development in 

 Onondaga and eastward counties where it reached a secluded 



