ON THE GASTEEOPODA OF THE GUELPH FORMATION OF CANADA. 543 



40. Notes on the Gasteropoda of the Guelph Formation of Canada, 

 By Henry Alleyne Nicholson, M.D., D.Sc, F.R.S.E., F.G.S., 

 Professor of Natural History in the University of St. Andrews. 

 (Eead June 23, 1875.) 



[Plate XXVI.] 



The " Guelph Formation " of Canada forms the uppermost portion 

 of the Niagara group of American geologists, and is composed of 

 dolomitic limestones, which have a thickness of about one hundred 

 and sixty feet. The establishment of the formation as a distinct 

 subdivision of the Niagara series is due to the labours of the Geological 

 Survey of Canada (Sir William Logan, ' Geology of Canada,' p. 336). 

 Subsequently Prof. James Hall recognized the occurrence of similar 

 magnesian limestones lying above the Niagara limestone at the 

 Leclaire rapids on the Mississippi river ; and he expressed the opinion 

 that they were identical with the Guelph formation of Canada 

 (Geology of Iowa, vol. i. p. 73). At a still later period, Professor 

 Hall detected limestones of the same mineral characters, holding the 

 same stratigraphical position, and charged with similar fossils, at 

 Racine and some other localities in Wisconsin (Report on the 

 Geology of Wisconsin, p. 67). Finally, beds of the same nature and 

 with the same organic remains have been recognized as forming the 

 summit of the Niagara series in Northern and South-western Ohio 

 (Report of the Geological Survey of Ohio, 1870, p. 277, and 1873, 

 p. 129). 



The lithological characters of the Guelph limestones, wherever they 

 may occur, are very constant. The characteristic rock of the forma- 

 tion is a rough, cellular, yellowish or creamy dolomite, of a rough 

 texture and crystalline appearance, exhibiting innumerable cavities, 

 from which fossils of various kinds have been dissolved out. The 

 organic remains which at one time existed in the rock must have been 

 extremely numerous ; but they are so preserved as rarely to show 

 any of the finer details of structure, and in the majority of instances 

 they present themselves only in the form of casts or moulds. Amongst 

 the characteristic fossils of the Guelph formation, in all or most of 

 the localities in which it has yet been detected, may be mentioned 

 Pentamerus occidentalis, Hall, various species belonging to the Trime- 

 rellidse, Megdlomus canadensis, Hall, corals belonging chiefly to Fa- 

 vosites and Anvplexus, and a numerous assemblage of Gasteropods be- 

 longing chiefly to the genera Murchisonia, Pleurotomaria, Subidites, 

 and Holopea. In some localities the formation has also yielded a 

 large number of Crinoids and Cystideans ; but these are hardly or not 

 at all represented in the formation as seen in Canada. Similarly, 

 Polyzoa, though very abundant in some localities, appear to be en- 

 tirely wanting in others, even over large areas. 



In the present communication I propose to make some observa- 

 tions on the Gasteropoda of the Guelph formation of Canada, so far 

 Q. J. G. S. No. 124. 2 o 



