218 



Shell much smaller than that of the preceding species, straight, though 

 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 in the largest and most perfect 

 specimen known to the writer, and somewhat compressed, so that the 

 outline of a transverse section in the broadest part would be elliptical or 

 ovately subelliptical : body-chamber truncated transversely at the anterior 

 end. Septate portion narrowly rounded at the apex in some specimens, 

 but more pointed in others, compressed conical and increasing rather 

 rapidly in size : body-chamber occupying about one-third of the total 

 length, and narrowing very gently and in some specimens somewhat con- 

 vexly towards and up to its anterior termination : aperture apparently 

 large, simple, open, and not much narrower than the posterior part of the 

 body-chamber : lip infolded. 



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 almost straight and parallel, the last two, at least, being coarsely 

 crenulated : siphuncle inflated between the septa, placed near the margin 

 and a short distance from the middle of one of the flattened sides. 



The dimensions of the largest specimen collected (the original of Plate 

 14, section 4, fig. 2, of the seventh volume of Transactions of the 

 Royal Society of Canada) are as follows: — length, 124 millimetres (or 

 nearly Hve inches) ; maximum breadth, seventy-four mm.; greatest diam- 

 eter from the siphonal to the antisiphonal side, fifty-eight mm. In the 

 specimen represented by fig. 3 on the same plate, the length along the 

 median line is sixty-six millimetres, the maximum breadth forty-seven 

 mm., and the greatest diameter fr6m the siphonal to the antisiphonal 

 side, forty mm. 



Dog Head, Lake Winnipeg, T. C. Weston, 1884, and D. B. Bowling,. 

 1891 ; Little Black Island, J. B. Tyrrell, 1889, and D. B. Bowling and 

 L. M. Lambe, 1890; and Cat Head, Messrs. Bowling and Lambe, 1890 : 

 a few specimens from each of these localities. 



This species possesses many characters that are common to it and to 

 the brevicone Orthoceratites for which Professor Hyatt has constituted 

 the genus Rizoceras, but it differs materially from that group or genus in 

 the circumstance that its body chamber always narrows distinctly from 

 its commencement to the aperture. From 7-". nohile it seems to be readily 

 separable by its much smaller size, more slender contour, and more com- 

 pressed sides. 



In Professor Blake's original description of P. intortum * (which, by 

 the way, seems to be very closely allied to P. apertum) the following 



* On page 187 of the first part of his "Monograph of the British Fossil Cephalopoda," 

 ondon, 1882. 



