AMERICAN PALEOZOIC CEPHALOPODS 
255 
the conch. The siphuncle is between the center of the conch and 
its ventral wall, and the segments of this siphuncle are more 
nearly circular or slightly elongate in outline. Poterioceras ven- 
tricosum McCoy apparently presents a different contour from 
typical Poterioceras, as based on fusiforme. 
45. AMPHICYRTOCERAS GEN. NOV. 
Genotype: Cyrtoceras orcas Hall 
Conch subfusiform, but curved lengthwise and depressed dorso- 
ventrally. The ventral side of the conch is curved convexly, 
and the dorsal side is concave except toward the upper part of 
the phragmacone and the lower part of the living chamber, along 
which it is gently convex or distinctly gibbous. The living 
chamber contracts toward the aperture, and the margin of the 
latter is nearly directly transverse dorsally and laterally, but 
curves gently downward toward the median line of the ventral 
side, where a shallow hyponomic sinus is located. The siphuncle 
is near the ventral wall of the conch, but not in actual contact 
with it. The septal necks curve distinctly downward, and the 
connecting rings tend to be almost cylindrical in form, except at 
their upper and lower extremities, where they contract abruptly. 
The genotype, Cyrtoceras orcas Hall, is relatively common in 
the Racine member of the Niagaran, at Racine, Waukesha, and 
elsewhere in Wisconsin. Cyrtoceras laterale Hall, from the same 
area and horizon, is congeneric. Oncoceras pettiti Billings, and 
possibly Oncoceras thales Billings from the Niagaran of Ontario 
also belong here. Strepoceras janus Billings and Streptoceras 
heros Billings differ in having a protruding hyponomic sinus, 
like an incipient lip. 
46. AMPHICYRTOCERAS ORCAS (HALL) 
PLATE XXIX, FIGS. 1 A, B, C 
Cyrtoceras orcas Hall; Rep. Superintendent Geol. Surv. Wisconsin, 1862, p. 43 
Oncoceras orcas Hall; 20th Rep. New York State Cab. Nat. Hist., 
1868, p. 350, pi. 17, figs. 1, 2 
The type of this species, measured along the curvature of its 
ventral side, is 143 mm. long. The living chamber appears to 
occupy 57 mm. of this length, but there is a possibility that one 
camera, whose upper septum is not exposed, is included in this 
measurement. The upper three exposed camerae occupy a 
length of 30 mm. ; the next three, 29 mm. ; and the lowest three. 
