219 



sentence occurs ; " The shell thickens near the aperture, but at last a 

 sudden inbending takes place to an opening much smaller than the 

 general section. This may, of course, be an abnormal feature." The thick- 

 ening of the shell towards the aperture and its "sudden inbending" are so 

 well shown in the two specimens of P. apertum from Little Black Island, 

 represented on Plate 11, section 4, of the ninth volume of Transactions 

 of the Royal Society of Canada, that these characters can scarcely be 

 considered as accidental or even abnormal. The original of fig. 2, on 

 this plate, is slightly and rather irregularly worn down in the siphonal 

 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 five segments of the very 

 eccentric siphunclp. 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 particular specimen the thick- 

 ening and inbending happen to be very slight. The specimen whose 

 aperture only is represented by fig. 3 on the same plate is so weathered 

 as to give a natural and longitudinal section of part of the shell, but the 

 section of 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 appreciably diminished in size by the infold- 

 ing of the lip. 



As stated elsewhere,* in the original description of this species, of 

 Oncoceras magnum, 0. gibhosum (Whit eaves not-Hall, = 0. Whiteavesii, 

 Miller) and Cyrtoc&ras Manitohense, their convex sides were inadvertently 

 designated as dorsal and their straighter sides as ventral, in accordance 

 with the old terminology. In the amended descriptions of these species 

 in this Report, the use of such terms as ventral, dorsal and the like is 

 purposely avoided, as it is not quite clear which is really the venter and 

 which the dorsum of either. 



PoTERiocEEAS GRACILE, Whiteaves. 



Poterioceras gracile, Whiteaves 1892. Trans. Royal Soc. Canada, vol. IX., 



sect. 4, p. 87, pi. 12, figs. 4 and 4, a-b. 



" Shell fusiform, strongly compressed, straight and rather slender, flat- 

 tened conical and obtusely pointed posteriorly, thickest at the mid- 

 length, where it is very gently convex, thence narrowing gradually and 

 very slightl}' towards the aperture, which apparently is simple and 

 broadly truncated ; immediately behind the aperture there is a faint 

 annular constriction : siphonal and antisiphonal regions narrowly rounded ; 



* In a foot note to page 102, sect. 4, of the eighth volume of Transactions of the Roya 

 Society of Canada. 



