ONONDASArSALT fiROUP. 



341 



galeatus, and numerous other silurian species which characterize the limestones at the base of 

 the Helderberg. And whether we regard them as of the age of the Ondondaga-salt group or 

 not, we know that they lie above the strata typified by the numerous fossils already described 

 as belonging to the Niagara group, and strictly should form no part of that group. It is true, 

 nevertheless, that in many places to the west of Cabot head, the Niagara group is very similar 

 in its lithological character to the Onondaga-salt group, though less argillaceous, and never 

 friable as some beds of the latter are. The species of Pentamerus figured on Plate 79 is appa- 

 rently identical with one which extends far westward, descending into the Niagara limestone j 

 and I presume it is the same species alluded to in the report of Mr. Murray to Mr. Logan, 

 as extending throughout the rock, but being more abundant near the top*. The same remark 

 may, however, be true of the Pentamerus oblongus, which at the west extends throughout the 

 limestone of the Niagara group, while within New-York it is restricted to the Clinton group, 

 and ia unknown above this positionf. 



706. 4. PENTAMERUS OCCXDENTALIS {n. sp.). 



PL. LXXIX. Figs. 1 a- s, and 2. 



Shell ovoid, more or less elongated, becoming ventricose with age ; surface marked by nu- 

 merous simple plications or strong strise, which are scarcely equal to the spaces between them ; 

 single plate of the dorsal valve extending more than half way to the base of the shell, becoming 

 thickened towards the beak ; plates of the ventral valve separated along their base, reuniting 

 only near the beak. 



All the specimens of this species which I have seen are more or less imperfect from the 

 removal of the shell, and even the best specimens have the outer part of the shell exfoliated. 



* Geological Survey of Canada : Report of progress for 1849 and 1849, page 16. 



f While the preceding p-,iges were passing through the press, I received from Mr. Logan, the Geologist of Canada, 

 a communication relative to the rocks at Gait, and enclosing the following memoranda from Mr. Murray, who has 

 particularly examined this district. 



"MoNTBKAij, Dec. 24, 1850. 



" With regard to the age of the group of recks which appear at Gait, and which Mr. Hall proposes to class as a 

 part of the Gypsiferous instead of the Niagara formation, this season's examination has tended to show that his sug- 

 gestion is founded upon correct data. 



"The rocks in question are extensively displayed on the Grand river, from Middleton bridge on No. 21, 6th 

 com. of Dumfries, all the way to the forks of the Speed above Preston ; at Guelph, upon the Speed; between Elora 

 and Fergus on the Grand river; and on the banks of the Rocky Sauquin. The fossil Mr. Hall proposes to call 



#MegaIomiis canadensis, was found in all these localities, but most numerous at Gait and at Elora, and in all cases 

 only among the upper beds of a group of limestone strata of peculiar character. Numerous spiral shells, among 

 which we supposed we could recognize the Loxonema hoydii and Euomphalus $ulcatv-», and numerous corals, were 

 tbund associated with the M. canadensis, and also in most of the lower beds of the group, especially at Elora, vrhere 

 there is a vertical section of the group exhibiting about SO feet. There is an undoubted difference in mineral as well 

 as fossil character, between these limestones and others on which they repose. The inferior rock in a dark brown, and 

 sometimes sdmost black, very bituminous limestone, interstratified with black bituminous shales; whereas the upper 

 one is of a pale yellowish or drab color, sometimes granular in structure, and apparently entirely free from bituminous 

 matter. The transition from the lower to the higher rock is well developed at Guelph on th« River Speed." 



