796 THE PALEONTOLOGY OF MINNESOTA. 
{Poterioceras apertum. 
Family GOMPHOCERATID &. 
Genus POTERIOCERAS, McCoy. 
PoTERIOCERAS APERTUM Whiteaves, 1889. 
PLATE LVII, FIG, 11. 
Poterioceras apertum WHITEAVES, 1889. Description of eight new species of fossils from the Cambro- 
Silurian rocks of Manitoba; Trans. Roy. Soc. Canada, vol. vii, sect. iv, p. 78, pl. XIv, 
figs. 2—4. 
Of three imperfect specimens the best preserved is a fairly satisfactory representa- 
tive of Whiteaves’ species, exhibiting the internal cast of the shell from the aperture 
to the eleventh septum (counting from the aperture) and conforming in size and 
other specific details with the originals. Though this specimen is considerably 
worn on one side, it shows very clearly that the venter is somewhat narrower than 
the dorsum and the aperture, narrowed by the contraction of the body-chamber, 
broad on the dorsum and sinused on the venter. These are both features which are 
more sharply developed in species of Oncoceras and some of the forms here referred 
to Cyrtoceras. Nevertheless the aspect of the shell is not that of either of these 
genera, and though recognizing the close relations in form of all these genera, we 
appreciate the usefulness of McCoy’s generic term, notwithstanding the fact that, 
as observed by Dr. Whiteaves, it has usually been assigned to the synonyms of 
Gomphoceras. 
The position of the sipho in all of our specimens is just within the margins of 
the right dorso-lateral surface (the shell being oriented with the venter toward 
the observer). Whiteaves describes its position as “a little nearer to the dorsal 
than to the ventral side,” but expresses at the same time a degree of uncertainty as 
to its proper place. 
The most complete of our examples measures 73 mm. in length; 24 mm. in 
dorso-ventral diameter at the first septum preserved (the eleventh from the aperture); 
43 mm. in the same dimension at the second septum from the aperture and this is 
the greatest width of the shell. The aperture is 836 mm. across. The body-chamber 
measures 30 mm. in length, and the eleven air-chambers cover 45 mm. One of the 
other fragments is larger, though less complete; the fifth septum has a dorso-ventral 
diameter of 44 mm., and here the diameter of the siphuncle is 8 mm. 
This shell has essentially the same proportions as the Gomphoceras [Poterioceras] 
cassinense Whitfield, from the Calciferous fauna at Fort Cassin, Vermont, but will 
be found to differ therefrom in its much shorter body-chamber*. 
Formation and locality.—In the lower blue beds of the Trenton limestone at Mineral Point, Wis- 
consin, and the Galena shales at St. Paul and Cannon Falls, Minnesota. 
Museum Register, No. 5837. 
* See Whitfield, Bull. Amer. Mus. Nat, Hist., vol. i, no. 8, p. 329, pl. xxrx, figs. 1—3. 
