78 WHITEAVES ON FOSSILS PEOM THE 



that Oncoceras, Hall, is syuonymous with Poterioceras, and states that Prof. Hall's " actual 

 type, as well as others which have been referred to the genus, in no respects differ 

 from M'Coy's genus." To this contention the present writer feels compelled to demur, 

 as the statement by which it is supported seems to be at variance with the known facts 

 of the case. If the genus Poterioceras is to bo retained at all, in the sense in which it 

 was defined by M'Coy, the name will probably have to be restricted to those straight, 

 GompJioceras-like shells in which the aperture is simple and entire, and it is in accordance 

 with tliis definition of its characters that the name will be used in the present paper. In 

 Oncoceras, on the other hand, the shell is always distinctly cirrved and inflated in a 

 peculiar manner in advance of the mid-length, while its body chamber is transversely 

 constricted jnst behind the aperture. According to Prof. Hall, the aperture of Oncoceras 

 is constricted, but as Mr. E. Billings asserts that it is oval,' the constriction referred to 

 by Hall may have been meant to refer to that immediately behind the aperture. 



In the outline drawing of the species on Plate XIY, although the contour and exact 

 dimensions of the original are correctly represented, the suturai lines are slightly restored 

 and the supposed outline of the aperture, on one side of the specimen, is indicated ])y a 

 dotted line. 



Poterioceras apertdm. (N. Sp.) 



(Plate XIV, figs. 2 to 4.) 



Shell much smaller than that of the preceding species, straight, thoiigh in some 

 specimens there is a scarcely perceptible curvature at the apical end, varying in contour 

 from ovately subfusiform to rather narrowly subovate, about one third longer than broad, 

 slightly compressed, but a little flatter on one side than the other, so that the outline of a 

 transverse section through the broadest part would be elliptical or ovately subelliptical, 

 the venter being sometimes slightly broader than the dorsum : body chamber truncated 

 anteriorly. Septate portion narrowly rounded at the apex in some specimens but more 

 pointed in others, and increasing rather rapidly in size, especially in the dorso-veutral 

 direction : body chamber occupying about one third of the entire leiigth, and narrowing 

 very gently and in some cases somewhat convexly towards and up to its anterior termi- 

 nation : aperture simple, open, not much narrower than the posterior part of the bodj"- 

 chamber, and narrowing apparently into a short, shallow and backwardly directed sinus 

 a little on or to one side of the middle of the venter. 



The surface markings are very imperfectly preserved, but the test appears to have 

 been nearly smooth and marked only with a few faint lines of growth. 



Sutures straight and parallel, the last two septa, at least, being coarsely crenulated ; 

 siphuucle inflated between the septa, placed near the outer margin, and a little on one 

 side of the centre of one of the flattened sides. 



The dimensions of the largest specimen collected (fig. 2) are as follows : — length, 124 

 millimetres (or nearly five inches) ; maximum dorso-A^entral diameter, seventy-four mm. ; 

 greatest lateral diameter, fifty-eight mm. In the other specimen represented on Plate XIV 



Geol. Surv. Can. Rep. Progr. 18,53-56, p. 311. 



