AMERICAN PALEOZOIC CEPHALOPODS 
251 
living chamber, along the middle of its lateral sides, is 20 mm. ; 
and the dorso-ventral diameter at this point is 20 mm., diminish- 
ing to 19 mm. at the aperture. 
This specimen is numbered 999B in the American Museum of 
Natural History, and is incorrectly associated under this num- 
ber with Oncoceras lycus. It was obtained at Beloit, Wisconsin, 
at the same locality and horizon with the type of Oncoceras 
plebeium. 
Oncoceras lycum Clarke^® appears to be closely similar to 
Oncoceras plebeium, as far as may be determined from the fig- 
ures. The increase in concave curvature of the dorsal side near 
the aperture and the straightening of the shell along the middle 
and lower part of the dorsal side of the living chamber is char- 
acteristic; the distance between the septa is similar ; also the 
course of the sutures. 
Cyrtoceras houghtoni Clarke^® may be the same as the smaller 
specimen figured by Whitfield under Oncoceras plebeium. In 
both figures 13 and 15 there is apparently a very faint trace of a 
straightening or swelling along the lower two-thirds of the 
living chamber. Moreover, the sutures of the septa are about 
the same distance apart. The extreme lateral compression of 
the conch shown by figures 14 and 14a is regarded as due to 
fossilization, and not as characteristic of the shell in its original 
state. 
42. BELOITOCERAS LYCUM (HALL) 
PLATE XXXVI, FIGS. 2 A, B 
Oncoceras lycum Hall; Rep. Superintendent Geol. Surv. Wisconsin, 
1861, p. 45 
Onoceras lycus Whitfield; Mem. Amer. Mus. Nat. Hist., vol. 1, 
pt. 2, 1895, p. 69, pi. IX, figs. 13, 14 
Specimen consisting of a living chamber with 10 camerae still 
attached. Conch moderately curved lengthwise in a dorso-ven- 
tral plane ; the radius of convex curvature of the ventral side of 
the phragmacone and of the lower half of the living chamber is 
55 mm. ; the radius of concave curvature of the dorsal side of the 
phragmacone is 40 mm. For a distance of 5 or 6 mm. below the^ 
base of the living chamber, and for an equal distance above the 
base, the dorsal outline of the conch is nearly straight lengthwise, 
suggesting a slight gibbosity of the dorsal outline of the lower 
Clarke, op. cit., p. 799, pi. 58, figs. l-3a. 
Idem. p. 807, pi. 59, figs. 12-15. 
