796 THE PALEONTOLOGY OF MINNESOTA. 



[Poterioceras apertum. 



Family GOMPHOCERATID^E. 



Genus POTERIOCERAS, McCoy. 

 POTERIOCERAS APERTUM Whiteaves, 1889. 



PLATE LVII, FIG. 11. 



Poterioceras apertum WHITEAVKS, 1889. Description of eight new species of fossils from the Carubro- 

 Silurian rocks of Manitoba; Trans. Roy. Soc. Canada, vol. vii, sect, iv, p. 78, pi. xiv, 

 flgs. 24. 



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 36 mm. across. The body -chamber 

 measures 30 mm. in length, and the eleven air-chambers cover 43 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 B^rt Cassin, Vermont, but will 

 be found to differ therefrom in its much shorter body-chamber*. 



Formation and locality. ID 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. 

 Bee WhltOeld. Bull. Anier. Mun. Nut. Hist., vol. I, no. 8, p. 320, pi. xxix, figs. 18. 



