THE TRENTON LIMESTONE OP MANITOBA. 87 



proportionate convexity of its dorsum and venter, and, more especially, in the much 

 smaller size and peculiar shape of its siphuncle. From the G. occidentale, Hall, of the 

 Trenton limestone of Wisconsin, it differs in the regularity of its lateral expansion, in the 

 nearly equal convexity of its dorsum and venter, in the absence of "retral curves" to the 

 septa, as well as in its very much smaller and differently shaped siphuncle. 



POTEEIOCERAS NOBILE. 

 Poteriociras nohile, Whiteaves. 1S90. Trans. Royal Soc. Can., vol. vii, sect. 4, p. 77, pi. xiv, fig. 1. 



So far as the writer is aware, no other examples of this species have been collected 

 than the two obtained by Mr. Weston in 1834 at East Selkirk and Lower Fort Garry. 



POTERIOCERAS APERTUM. 



Plate XI, figs. 2 and 3. 

 Pokrioceraa apcrlum, Whiteaves- 1890. Trans. Eoyal Soc. Can., vol. vii, sect. 4, p. 78, pi. xiv, figs. 2-4. 



Since this species was described, a few additional specimens have been collected by 

 Messrs. Bowling and Lambe, in 1890, at Cat Head, Lake Winnipeg, and at Black Island, 

 near Swampy Island ; also by Mr. Bowling, in 1891, at Bog Head. 



In Professor Blake's original description of P. intorlum (which, by the way, seems to 

 be very closely allied to P. apert/um) the following sentence occurs : " The shell thickens 

 near the aperture, but at last a sudden iubeuding takes place to an opening much smaller 

 than the general section. This may, of course, be an abnormal feature." The thickening 

 of the shell tow^ards the aperture and its " sudden inbending" are so well shown in the 

 two specimens of P. aperlum from Black Island, represented on Plate XI, that these char- 

 acters can scarcely be considered as accidental or even abnormal. The original of fig. 2, 

 on Plate XI, is slightly and rather irregularly worn down in the siphonal or ventral 

 region, in such a way as to give a natural and longitudinal section of the whole shell, 

 very near to the surface. Posteriorly, the weathering of this specimen exposes six or 

 seven of the septa and live segments of the very eccentric siphuncle. Anteriorly, it gives 

 a section of the whole of the body chamber, and, more particularly, of the thickening and 

 inbending of the test at the aperture, though in this particiUar specimen the thickening 

 and inbending happen to be very slight. The specimen whose aperture only is repre- 

 sented by fig. 3 on Plate XI is so weathered as to give a natural and longitudinal section 

 of part of the shell, but the section at the aperture is nearly through the centre of the 

 latter. In this specimen the thickening and infolding of the test at the aperture are 

 strongly marked, the test being fully six millimetres thick at its recurved extremity, and 

 the aperture itself appreciably diminished in size by the infolding of the lip. 



POTERIOCERAS URACILE. (Sp. Nov.) 

 Plate XII, figs. 4 and -Ja. 

 Shell fusiform, strongly compressed, straight and rather slender, flattened conical and 



