CEPHALOPODA. 795 
Gonioceras occidentale. ] 
GONIOCERAS OCCIDENTALE Hall, 1861. 
PLATE LVII, FIG. 6. 
Gonioceras occidentalis HALL, 1861. Rept. Supt. Geol. Sury. Wisconsin, p. 47. 
Original description: “Shell elongate, very compressed, extremely expanded 
laterally, the upper part with curved outline, beyond the middle the edges are more 
nearly parailel; the length (when entire) having been a little less than twice the 
greatest diameter. Upper and lower surfaces convex, the one twice as convex asthe 
other; the two diameters as one to seven; lateral expansions very thin. Septa deeply 
concave, numerous, closely arranged, twelve to the inch in the central lobe; arching 
forwards on the sides with a sharp retral curve a little within the margin, and 
running backwards in a narrow extension to the edge at a point opposite or below 
their junction with the siphunele in the central lobe. Siphuncle oblate [ventral] of 
medium size where passing through the septa, expanding in the chambers to more 
than one-half the smaller diameter of the shell, somewhat bilobate from a constric- 
tion above and _ below. 
“Surface apparently smooth, or with only concentric lines of growth.” 
The principal characters distinguishing this from the foregoing species will 
be found in the closer septa of the former and the curvature of the septa on the 
lateral expansions. The latter feature is not sufficiently emphasized in the quoted 
description. ~ 
In G. occidentale these saddles are quite regular, the outer and inner slopes 
together making almost the arc of a circle or the extremital arc of a broad ellipse, 
but in G. anceps the saddles do not rise above their height at the junction of the 
lobes with the body of the shell, whence they are deflected backward in a long, 
broad curve. The species seem to agree in the general form of the shell and the 
size of the apical angle. The best preserved specimen fails of agreement with the 
description in the proportional dimensions of the shell, the minor and major 
diameters being here as 1 to 5, rather than as 1 to 7. The latter ratio would make 
a much more expanded form than that presented by our specimens. 
Formation and locality.—One considerable fragment and two quite imperfect examples are from 
the Trenton limestone at Dixon, I)linois (collected by E. O. Ulrich). The original locality is in the Tren- 
ton at Platteville, Wisconsin. 
