ARCTIC ORDOVICIAN AND SILURIAN CEPHALOPODS 279 
median part of the shell the curvature of the sutures is evenly 
concave, but toward the lateral sides they tend to diverge at 
angles of 155 to 158 degrees. The curvature of these sutures 
corresponds approximately to that which would result if a nor- 
mal cylindrico-conical Orthoceroid had one side ground away 
until a similar subtriangular cross-section were produced. The 
sutures curve downward a distance of 6 or 7 mm., reaching their 
lowest level about 7 mm. dorsad of the center of the siphuncle. 
At the lower end of the specimen where the dorso-ventral dia- 
meter of the conch is 30 mm., the narrowest part of the septal 
necks is slightly less than 5 mm. in diameter. The length of 
these septal necks is 2 mm. and at their lower margins they curve 
distinctly outward. Within the camerae the segments of the 
siphuncle attain a maximum diameter of 9.5 mm. In vertical 
sections the outlines of these segments tend to be circular rather 
than broadly nummulitic, at least along the lower part of the 
specimen. The shell material of the siphuncle appears to have 
been extremely thin, and only parts of the connecting rings are 
distinctly preserved, in some camerae more than in others. 
No indication of surface markings is retained by the specimen, 
which is a cast of the interior of the conch. 
Locality and Horizon. — From some unknown locality either 
on Boothia Felix or on King William Land; probably from Black 
River limestone. In the Palaeontologisk Museum, Kristiania, 
Norway. Collected in 1903-04 by Lieut. Godfred Hansen in 
whose honor this species is named. 
Remarks. — Leurorthoceras hanseni is very similar to Leurortho- 
ceras chidleyense, described in this paper, from which it differs in 
having a smaller apical angle; it is less strongly flattened ven- 
trally, and the segments of the siphuncle are relatively narrower. 
Both species are congeneric and both are regarded as belonging 
to a new genus for which the term Leurorthoceras is proposed, 
with Leurorthoceras hanseni as the genotype. This is charac- 
terized by the flattening of the ventral side of the conch, the 
broad ventral lobes of the sutures, and the relatively moderate 
inflation of the segments of the camerae compared with strongly 
nummulitic forms like Actinoceras, 
