TEE DEVONIAN LIMESTONE AT ST. GEORGE, QUEBEC 221 



as our specimens show all gradations between such a form and 

 another in which the hinge Kne exceeds the greatest width of the 

 valves, I have assumed that this variation is of minor importance. 

 The significance of the association of this species, which in Ohio 

 occurs in beds of Hamilton age, with true Onondaga corals, is dis- 

 cussed below. This is probably the species identified by BiUings, 

 Ami, and Kindle as S. gregarius Hall. Stauffer has suggested that 

 two of Hall's figured specimens of that species^ may belong to 

 S. lucasensis. 



GASTEROPODA 



Igoceras cf. conicum (Hall) 

 Igoceras cf. plicatum (Conrad) 



Fragmentary representatives of this genus were collected at 

 St. George, and tentatively assigned to the species suggested above. 



THE AGE OF THE LIMESTONE 



There is an apparent contradiction in the evidence presented by 

 the fossils. All of the corals definitely identified are typical Onon- 

 daga species, and as they far outnumber, both in species and in 

 number of individuals, the representatives of other phyla, we may 

 assume for them a higher diagnostic value than for the other 

 fossUs. In fact, there is but one other fossil with any diagnostic 

 value in our collection — Spirifer lucasensis, which has previously 

 been reported from the Delaware formation (the equivalent of the 

 Hamilton and Marcellus beds) of Ohio. The Columbus limestone, 

 which underlies the Delaware formation, is rich in corals, and is the 

 western equivalent of the Onondaga. Corals do occur in the Del- 

 aware, but they are not characteristic of that formation. Since 

 the Delaware postdated the Onondaga of Quebec, S. lucasensis is 

 evidence of a faunal migration westward near the end of Onondaga 

 time. At about this time the St. Lawrence channel was per- 

 manently closed, and a retreat of the waters from the maritime 

 provinces westward ensued. Such a retreat could not promote a 

 continuance of the clear waters which the Onondaga fauna predicate; 

 in fact, in the sediments immediately succeeding this event we have 



' James Hall, Paleontology of New York, Vol. IV, Pt. I, PI. 28, Figs. 9-10. 



