254 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1915. 



disclosed Olenopsls roddyi on the eastern side of the continent, near 

 Lancaster, in the central part of Pennsylvania; on the weslern side 

 of the continent Olenopsis americanus in the northern central part of 

 Montana, and Olenopsis ? agnesensis on the line of the Continental 

 Divide, near the Canadian Pacific Railway, in both Alberta and 

 British Columbia. It is quite probable that if entire specimens of 

 a number of species now represented by cranidia and referred to 

 the genus Ptychoparla were available for study other species of 

 Olenopsis would be found at approximately the same stratigraphic 

 horizon. 



AN EARLY DISCOVERY BY THE AUTHOR. 



Another instance of settling a disputed horizon ^ recalls a per- 

 sonal experience. In a small drift block of sandstone which I found 

 in 1867 on the road from Trenton to Trenton Falls, Oneida County, 

 N. Y., there is an unusual apparent association of Upper Cam- 

 brian (Hoyt limestone) and Ordovician (Aylmer sandstone, Chazy) 

 fossils. The Hoyt limestone species are Ptychaspis speciosus and 

 Agraulos cf. saratogensis. The Aylmer sandstone species are Leper- 

 ditia arniata and Bathyurus cf. angelina Billings. 



When as a boy I found the rounded block of sandstone referred 

 to I broke out all the fossils possible, as at the time I was well 

 acquainted Avith the Trenton limestone fauna, and the fossils in the 

 block Avere strangers to me, with the exception of Leperditia armata. 

 The following winter I endeavored to locate the stratigraphic posi- 

 tion of the trilobites, but could not, further than that they were 

 evidently of pre-Trenton age. This study aroused an interest in 

 the American early Paleozoic fossils that gradually led me to take 

 up the Cambrian rocks and faunas as my special field of research. 



The block of sandstone I had found was about 3 inches in thickness 

 by 12 in diameter. The impact of the wheel of the wagon in which 

 I was riding split the block open and exposed several cranidia of 

 the trilobite now known as PtycJioapls speciosus. Neither this nor 

 Agraulos cf. saratogensis occurred in direct association with the 

 Chazy Leperditia, and Bathyurus. 



In explaining this connection I have recently been led to adopt a 

 suggestion of Dr. E. O. Ulrich that the block of sandstone Avas part 

 of a bed formed by the overlap of the Aylmer sandstone of the Chazy 

 on a layer of Potsdam sandstone. This would make the line of 

 demarcation between the Cambrian and Ordovician deposits within 

 the block of sandstone that I found. With this view in mind, the 

 Hoyt limestone species have now been referred by me to the Upper 

 Cambrian and the Aylmer sandstone species to the Ordovician. 



* Walcott : New York Potsdam-Hoyt fanna, Smithsonian Misc. Coll., vol. 57, No. 9, 1912. 



