316 THE CAMBRIAN. [BUiiu8t. 



or less abundant, but especially so near the summit where there were 

 obtained the following : 



Liugulepis majra. 



minuta. 

 Lingula ? manticula. 

 Obolella discoidea. 

 Acrotreta gemma. 

 Kutorgina minutissima. 

 Agnostus communis. 



bidens. 



neon. 



prolongus. 



tumidosus. 



tamifrons. 

 Dicellocephalu8 ? nasutus. 



Dicellocephalus angustifrons. 



marica. 



bilobus. 



oseeola. 

 Ptychoparia affinia. 



oweni. 



haguei. 



granulosa. 



simulator. 



unisulcata. 



breviceps. 

 Arethusina americana. 

 Ptychaspis minuta. 



"Three of these species, Hyolithes primordialis, Dikelocephalus oseeola 

 and Ptychaspis minuta, are identical with forms from the Potsdam sand- 

 stone of Wisconsin." x As a whole the fauna is of the general facies of 

 that of the Potsdam zone of New York and the Mississippi Valley. 

 This is found in the Hamburg shale, but in the overlying Pogonip lime- 

 stone there is an assemblage of species that combine both Cambrian 

 and Lower Silurian (Ordovician) types. A short distance above, the 

 fauna is comparable to that of the Calciferous and Ohazy groups of the 

 New York section. 



The transition from the Cambrian to the Silurian [Ordovician] fauna is very grad- 

 ual, and such as would occur where there was no marked physical disturbance to in- 

 fluence the faunal change resulting from the natural dying out and development of 

 species or the influx of new species from other areas. 



Of the species occurring below the passage beds three are identical with species oc- 

 curring in the Potsdam sandstone of Wisconsin, viz: tIyolithe-8 primordialis, Dicelloce- 

 phalus oseeola, and Ptychaspis minuta; one, with A crotreta gemma of the Calciferous 

 formation of Newfoundland; and Ptychoparia oweni is a common species of the Pots- 

 danl horizon in Montana and Dakota. These specific identifications and the great 

 development of species of the genera Agnostus, Dicellocephalus, and Ptychoparia in 

 the middle and upper portion of the Cambrian section, furnish abundant evidence 

 upon which to correlate the fauna and the geologic horizon at which it occurs with 

 the Potsdam fauna and formation, as was done by Messrs. Hall and Whitfield (Geol. 

 Expl. Fortieth Par., vol. 4, 1877, p. 199). Of the Potsdam fauna eleven genera and 

 fifteen species continue on into the passage fauna, viz : (species of the Potsdam fauna 

 are printed in italics); Lingulepis mcera, L. minuta, Lingular manticula, Discina (sp. 

 undt.), Acrotreta gemma, Schizambon typicalis, Obolella ambigua, 0. discoidea, Lep- 

 tarna melita, Orthis hamburgensis, O. testudinaria, Triplesia calcifera, Tellinomya? 

 hamburgensis, Agnostus communis, A. bidens, A. neon, Dicellocephalus fiualis, D. in- 

 expectans, Ptychoparia? annectans, Ptychoparia affinis, P. granulosus, P. haguei, P. 

 oweni, P. unisulcatus, Arethusina americana, Amphion (sp. undt.) BarrancTia maccoyi, 

 Illieuurus eurekensis, Asaphus caribouensis. 



In the next superior grouping, about midway of the Pogonip group, all the middle 

 Cambrian genera, with the exception of Orthis and Illaenurus, have disappeared, and 

 higher up the genera Receptaculites, Chtetetes, Pleurotomaria, Maclurea, Cyphasis, 

 Bathyurus, and Asaphus carry the fauna up to the summit of the formation where the 

 genera Receptaculites, Ptilodictya, Chaetetes, Strophomena, Orthis, Tellinomya, 



*0p. cit.,p. 33. 



