AECTIC ORDOVICIAN AND SILURIAN CEPHALOPODS 267 
eric value of the differences noted above, the Ellesmereland 
specimen here described does not appear to me to be a typical 
Endoceras; the absence of any trace of endocones is certainly 
generic, if constant; hence the new generic term Ellesmeroceras, 
with Ellesmeroceras scheii as the genotype is here proposed. 
Conchs with siphuncles in contact with the ventral wall are 
fairly common in the Canadian strata of eastern Canada and 
in the adjacent parts of the United States. Most of these forms 
are smooth but some are annulated. Most species are straight 
but curved forms also occur. Among smooth forms with 
relatively small siphuncles may be mentioned the species des- 
cribed by Billings as Orthoceras cato, 0 . repens, and 0 . perseus. 
The latter is laterally compressed. Orthoceras sordidum may 
belong here also. In Orthoceras autolycus the conch is slightly 
curved lengthwise. 
Conchs with siphuncles in contact with the ventral wall occur 
also in the Ozarkian but most of these are more or less distinctly 
curved lengthwise. 
The number of these early cephalopods in which the segments 
of the siphuncle show concave lateral outlines also is of interest. 
This type of outline is noted in Eremoceras syphax and Elles- 
meroceras scheii. It is seen also in Cyclostomiceras cassinense 
(Whitfield). It is figured by Ruedemann in Orygoceras cornu- 
oryx (Whitfield), Protocycloceras whitfieldi Ruedemann, and 
Endoceras montrealense (Billings). It was noted by Billings in 
the species described by him as Orthoceras menelaus, Orthoceras 
indegator, and Orthoceras flavins, the last two of which have 
the siphuncle in contact with the ventral wall, as in Endoceras 
montrealense. The number of such Canadian and Ozarkian 
cephalopoda with siphuncular segments presenting vertical 
lateral outlines is considerably multiplied if cyrtoceraconic forms 
be included. This type of outline appears to occur chiefly in 
Holochoanitic forms, but a reexamination of all forms having 
this type of siphuncle is necessary. 
As far as can be determined from the preceding observations, 
the occurrence of Clarkoceras holtedahli and Ellesmeroceras 
scheii in the Orthoceras limestone at Victoria Head suggests that 
this limestone includes a horizon of Canadian age. 
