AMERICAN PALEOZOIC CEPHALOPODS 
199 
number of specimens which expose well the interior of the si- 
phuncle is relatively small. However, when the genus Ellesmereo- 
ceras was described, the absence of endocones was regarded as 
one of its characteristic features. 
Orygoceras is another orthoconic Canadian genus with si- 
phuncular segments presenting concave vertical outlines. The 
conch displays a smooth exterior, but an annulated interior. 
Canadian cephalopods are remarkable for the number of 
genera and species in which the segments of the siphuncle .pre- 
sent concave vertical outlines. Such outlines are characteristic 
of the holochoanitic cephalopods, in which the septal necks are 
prolonged downward at least the height of one of the camerae. 
In Zittel-Eastman’s Text-book of Paleontology the Holochoan- 
ites include the genera Vaginoceras, Cameroceras, Endoceras, 
Narthecoceras, Nanno, Piloceras, and Cyrtendoceras. To this 
list the genera Cyclendoceras and Clarkoceras have been added 
by Bassler.^ Ruedemann recognized the holochoanitic structure 
of the siphuncle of Protocycloceras,^ his illustrations show the 
concave vertical outlines of the siphuncular segments of Cyclo- 
stomiceras. His vertical section of Orygoceras indicates a 
similar structure. The present writer has described the concave 
vertical outlines of the siphuncular segments of Eremoceras and 
Ellesmereoceras, and in the present bulletin he calls attention to 
similar structure in various orthoconic cephalopods described by 
Billings from Canadian strata. 
Although it does not follow necessarily that species with si- 
phuncular segments having concave vertical outlines are holo- 
choanoidal in structure, the writer believes that such a relation- 
ship exits. Should this prove to be true, after further investiga- 
tion, then it follows that the chief characteristic of Canadian or- 
thoconic and cyrtoconic cephalopods is the remarkable dominance 
of holochoanitic forms. 
Although such holochoanitic genera as Cameroceras, Cyclen- 
doceras, Endoceras, Narthecoceras, Nanno, Suecoceras, and 
Vaginoceras are well represented in Ordovician strata, they here 
are associated with numerous Orthochoanitic and Cyrtochoanitic 
forms which are either rare or absent in Canadian strata. The 
^ Bassler, R. S., Bibliographic Index of American Ordovician and Silurian 
fossils; U. S. Nat. Museum Bull. 92, 1915. 
® Ruedemann, R., Cephalopoda of the Beekmantown and Chazy forma- 
tions of the Champlain basin; New York State Museum Bull. 90, p. 441, 1906. 
