CAMBEO-SILUEIAN OP MAN1T015A. 79 



(fig. 3), the length aloug the median line is sixty-six millimetres, the maximum dorso- 

 ventral diameter forty-seveu mm., and the greatest lateral, forty. 



Dog's Head, Lake Winnipeg, T. C. Weston, 1884, and Swampy Island, in the same lake, 

 J. B. Tyrrell, 1889 ; three nearly perfect ])ut badly preserved specimens from each of these 

 localities. 



The siphuncle seems to have been placed a little nearer to the dorsal than to the 

 ventral side, but the only specimen iu which any remains of this part of the sliell can be 

 detected is so much worn that its relative position is uncertain. 



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

 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 P. nobile it seems to be 

 readily separable by its much smaller size, more slender contour, and more compressed 

 sides. 



Oncoceras magnum. (N. Sp.) 



(Plate XV, fig. 1.) 



Shell very large, attaining a length of at least eight inches,' slightly but distinctly 

 curved, somewhat fusiform but much more convex on the dorsal than on the ventral side, 

 and contracted at both euds ; sides compressed, though one side is flatter than the other, 

 so that the outline of a transverse section through the thickest part would be nearly ovate, 

 the dorsum being broader than the venter ; maximum dorso-ventral diameter about twice 

 as great as the lateral, and nearly equal to one half of the entire length. Septate portion 

 increasing rather slowly in size from the posterior end to a little in advance of the mid- 

 length, after which it narrows somewhat more rapidly, especially on the dorsal side ; its 

 dorsal margin not only uniformly convex, but distinctly gibbous anteriorly, while its 

 ventral margin is shallowly concave behind and slightly convex in front. Bodj^ chamber 

 oblique, short and occupying about one third the entire length ; at its commencement pos- 

 teriorly, it is broad, especially in a dorso-ventral direction, and bounded by a deep oblique 

 groove, which is broader on the dorsum than on the venter, and parallel to the septa 

 nearest to it ; iu front of this groove the chamber narrows rapidly, but at first convexly, 

 towards the aperture, behind which there is a broad and shallowly concave constriction. 

 In the most perfect specimen known to the writer, the characters of the ajierture are very 

 imperfectly shown, but it appears to have been simple, entire, rather narrow and subovate, 

 with its lateral margins obliquely truncated ; there are also some indications that it 

 narrowed into a short sinus on the ventral side. 



Sutures oblique, the septa being broader on the dorsum than on the venter, nearly 

 straight on the sides but faintly convex on the dorsum, and slightly concave on the venter ; 

 siphuncle inflated between the septa, nummuloidal, eudogastric, and placed very near to 

 the ventre, but not quite marginal. 



Surface markings unknown. 



' In an imperfect and water-worn cast of a shell collected by Mr. Tyrrell at Swampy Island this summer, which 

 is probably referable to this species, the length is fully eleven inches. 



