AMERICAN PALEOZOIC CEPHALOPODS 
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1. THE HOLOCHOANITIC CEPHALOPODS OF CANADIAN STRATA 
Ulrich included in the Canadian series of strata the Beekman- 
town of New York, Vermont, and adjacent Canada, the Levis 
of the Quebec area, the Romaine of the Mingan island area, all 
but the basal part of the Bretonian of Nova Scotia, the upper 
part of the Quebec group of Newfoundland, and the approxi- 
mate equivalent of these strata elsewhere. Those strata which 
intervene between the Canadian and the typical Cambrian he 
included in the Ozarkian. The fauna of the Potsdam standstone 
is regarded as distinct from the typical Cambrian and as char- 
acterizing the basal part of the Ozarkian. 
The cephalopod fauna of the Ozarkian and Canadian differs 
so strongly from that of the overlying Ordovician that it sug- 
gests that the interval between the Canadian and the Ordovician 
is one of the great time-breaks of geology. Very few genera 
bridge the interval between the Canadian and Ordovician, and 
these few are represented in the Ordovician by species which 
usually are strikingly different from those in the Canadian. 
Such genera as Eurystomites, Tarphyceras, Aphetoceras and 
Trocholitoceras are, strictly speaking, Canadian. It is possible 
that an intensive study of Eurystomites plicatus and Eurysto- 
mites rohertsoni will reveal differences from the typical Ca- 
nadian forms of sufficient importance to be at least of subgeneric 
rank. In Eurystomites plicatus, for instance, the segments of 
the siphuncle enlarge slightly within the camerae, as though 
short septal necks with intermediate connecting rings were pres- 
ent, while this appearance is not noted in Canadian species. In 
Tarphyceras multicameratum, moreover, the camerae are much 
shallower and the rate of increase in diameter of the conch is 
greater. Nothing corresponding to the Canadian species of 
Schroederoceras occurs in the Ordovician of North America. 
Typical Schroederoceras, of course, is European, 8 European 
species being recognized by Hyatt. Typical Trocholites is of 
Trenton or Cincinnatian age, but Trocholites internistriatus 
(Whitfield) occurs as early as the Beekmantown division of the 
Canadian. 
The Canadian species described by Ruedemann under the term 
Cyrtendoceras (?) priscum differs from anything known in the 
Ordovician of America. 
Canadian as well as Ordovician and Silurian species have been 
