224 The American Geologist. October, looi. 
The decreased number of species in the Waverly may there- 
fore not he of much sig"nificance, but the evident fact of a pro- 
gression forward from the Corniferous, the unquestioned 1am- 
elHbranchiate evohition of increased numbers and species to 
the Chemung- and the obvious gaps between the Corniferous 
and Hamikon, the Hamilton and Chemung, the Chemung and 
Waverly are. 
Such gaps are an evident indication of a suspension of 
niolluscan life incident upon unfavorable conditions and the 
withdrawal of the previous molluscan faunas elsewhere, as 
there can be no doubt of the continuity of this lamellibranchiate 
life through the Upper Helderberg, Hamilton and Chemung 
periods. Those conditions were shore conditions, shallow 
water, continental drainage and more or less disturbed marine 
tracts wherein storms or waves easilv dislodged shells, and 
caused their withdrawal to deeper water. On the other hand 
the increase of species in the populous periods after these gaps, 
would seem to indicate an acceleration in the production of 
species, which must have begun in some areas to which the 
molluscan fauna retreated upon the invasion of their previous 
habitat by the conditions represented in the Marcellus, Gen- 
essee. Portage and Catskill. 
The Marcellus group certainly does not represent a period 
totally inimical to molluscan life. It only marks a decresence 
of the previously crowded fauna of the Upper Helderl^erg and 
holds within it (Clarke, Livonia Salt Shaft, 13th Annual Re- 
port, N. Y. State Geologist, p. 156-158), a succession of faunas 
which are lioth reminiscent and prophetic, being Helderbergian 
and Hamiltonian in nature, besides its own indigenous fossils. 
The Marcellus, as the Genessee, presents, however, an in- 
structive illustration of displacement and conflict. It intro- 
duced conditions, and brought with it a life contrasted w^ith the 
conditions and life which marked the Upper Helderberg, and 
yet in those conditions and life there remained a struggling 
survival of the latter, and many of the lamellibranchiate genera 
pushed their way through to the Hamilton, while on the other 
hand the Hamiltonian fauna moving landward inserted itself 
'' prciuinciaily" (Clarke) in the Marcellus areas. 
Similarly in the Genessee, outside of the fundamentally 
characteristic '^iiifiinicscciis" faiuia, the relics of Hamilton life, 
