194 
AUG. F. FOERSTE 
Such holochoanitic genera as Encloceras, Camerocey^as and Pilo- 
ceras have siphuncles built by successive septal necks which ex- 
tend downward until each invaginates into the neck next beneath. 
Within the camerae this type of neck generally has a concave 
vertical outline. The septal necks of Clarkoceras, Eremoceras, 
Ellesmereoceras and Protocycloceras^ have vertical outlines simi- 
larly concave. Ruedemann has shown corresponding structures 
in Protocycloceras, Orygoceras and Cyclostomiceras.^ A recent 
examination of all the orthoconic and crytoceraconic species de- 
scribed by Billings from the Canadian, indicates that concave 
vertical outlines prevail in all the specimens which reveal the 
structure of the siphuncle. The typical Black River form of 
Cyrtocerina has an ellipochoanoidal structure, but the siphuncle 
of the Canadian species Cyrtocerina mercurius, presents seg- 
ments with concave vertical outlines, thus indicating that it is 
really a Clarkoceras. In the numerous species of '' Orthoceras” 
described by Billings, concave vertical outlines prevail along the 
segments of the siphuncle within the camerae. 
Apparently the structure of the Canadian orthocones and cyr- 
toceracones is prevailingly that of the holochoanitic cephalopods. 
Corresponding ellipochoanoidal genera make their appearance 
in Chazyan time, though such holochoanoidal genera as Endoceras 
continue through that epoch into later stages. If these observa- 
tions are verified by increasing knowledge of Ozarkian and Ca- 
nadian cephalopods, they would suggest that the interval between 
the Canadian and Chazyan stages was one of the most important 
in geologic history, sufficient to admit of a major change in the 
structure of cephalopods. If corresponding changes are found in 
other groups of animals, contrasting those of Canadian age with 
those of Chazyan and later times, the separation of Canadian 
strata from the Ordovician system would seem as fully warranted 
by paleontology as by stratigraphy. 
The enormous number and variety of species which have been 
referred to the genus Orthoceras has always been a serious prob- 
lem to the systematist. Barrande grouped his numerous species 
into various divisions based on surface ornamentations. Hyatt 
has proposed generic names for the major groups. Later, Hyatt 
^ Foerste, A. F., Notes on Arctic Ordovician and Silurian cephalopods; 
Journ. Sci. Lab. Denison Univ., vol. 19, pp. 261-271, 1921. 
^ Ruedemann, R., Cephalopoda of the Beekmantown and Chazy formations 
of the Champlain basin; New York State Museum Bull. 90, pp. 444, 451, 
502, 1906. 
