304 
AUG. F. FOEKSTE 
is made to enlargement of the segments of the siphuncle within 
the camerae, these segments may be regarded as cylindrical in 
form. From the cast it appeared possible to secure a transverse 
section of the whorl (at the point B in fig. 9A on Plate XXXIII of 
this paper) which is angularly elliptical (as in fig. 9B of the plate 
just cited), but the excellent cross-section presented by Dr. 
Bather removes all doubt about the outline of the whorls. Later- 
ally, the sutures of the septa are gently concave when viewed 
from above, but nothing is known of their direction along the 
ventral side of the conch. 
I do not know of any Silurian nautiloid combining the charac- 
teristics here noted. Among Ordovician forms the cast of the 
interior of the some species of Plectoceras present a somewhat 
similar appearance on lateral view. However, the Wellington 
channel specimen is not known to have its sutures curving 
distinctly downward, along the ventral side of the conch, as in 
typical Plectoceras. • The absence of any indication of obliquely 
transverse plications on the ventro-lateral sides of the specimen 
is not sufficient to exclude it from Plectoceras, since casts of 
the interior of some species of this genus do not show readily 
recognizable undulations, even when these are quite boldly 
marked on the exterior. The shell of typical Plectoceras is 
so thick that the interior may be quite smooth even in the 
presence of a strongly ribbed exterior. 
I do not know of any Silurian species of Plectoceras. Plecto- 
ceras jason and Plectoceras tyrans were described by Billings from 
the Chazyan of the Mingan Islands, in northeastern Canada. 
Plectoceras hondi was described by Safford from the Stones 
River of central Tennessee. Plectoceras undatum described by 
Conrad from the Black River at Watertown, New York. Plecto- 
ceras occidentale was described by Hall from the Platteville of 
Wisconsin. Plectoceras halli was described by Foord from the 
Black River near Quebec, in Canada. The Wellington Channel 
species described by Foord under Trochoceras horeale may not 
belong to Plectoceras. Its affinities may be with Eurystomites, 
in which the shell is not conspicuously cosiated. This would 
place it among the Tarphyceratidae. 
