338 H. S. WILLIAMS — SILURIAN-DEVONIAN BOUNDARY 



the species on passing from the top of the Silurian to the first faunas of 

 the Devonian of that region. 



In the typical Welsh region the conspicuous geological event marking 

 the passage from Silurian to Devonian is the change from marine to 

 fresh-water conditions and sedimentation, the latter being represented 

 by the Old Red sandstone. This passage was associated with the ap- 

 pearance in the Ludlow of fish and merostomes, and those who have 

 based their opinions on the New York and interior sections of American 

 rocks have assumed that the place of prominent appearance of Euryp- 

 terids (viz, the Waterlime), is equivalent to the zone of the " Tilestones" 

 of Europe. 



An examination of the Maine, New Brunswick, and Gaspe sections 

 facing toward the Atlantic, and much nearer to Wales, brings out clearly 

 the invalidity of this interpretation. In this eastern region of America 

 the gradual emergence of land, affecting the marine faunas and finally 

 terminating the marine faunas for Paleozoic time, was of a similar nature 

 to that on the opposite shore of the north Atlantic. Furthermore, the 

 exact stage of the physical transition is at a similar point in the evolu- 

 tion of its marine organisms. 



MEROSTOMES AND FISHES 



The Merostomes, as Walcott has shown, were represented in the Utica 

 (Echinognathus) and possibly in still more ancient pre-Cambrian time. 

 So, too, the fishes are not appearing for the first time in the Ludlow ; 

 but, as again Walcott has shown, they were well developed in Trenton 

 time (Astraspis, Eriptychius, etcetera, from Canyon City, Colorado). 



Hence the argument based on supposed identity of the Waterlime 

 with the top of the Ludlow because of the appearance in both of mero- 

 stomes and fishes loses its force. We are obliged from the evidence to 

 believe that throughout Silurian time both fishes and merostomes were 

 in existence. They are liable to occur at any place in the system where 

 the conditions of their living were represented by the formation of strata 

 in which the faunas could be preserved. 



SALT BASINS OF WATERLIME AND ONONDAGA OF THE INTERIOR AND THEIR 

 ABSENCE IN MAINE AND NEW BRUNSWICK SECTIONS 



In the New York and interior regions there were conditions which shut 

 out, locally, the sea, and in these brackish water pools fish and mero- 

 stomes appeared; but a comparison of the marine faunas of these re- 

 gions, below and above the Waterlime, proves that the local elevation, 

 which is expressed in the Onondaga and Waterlime, as well as in the 

 Guelph and Gait, was quite distinct from and at a much earlier geo- 



