30 TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE. [VOL. xI 
second camera, from which point the shell contracts anteriorly, but 
presents a sharp outward reflexion with crenulation at the aperture. 
The measurements of this specimen are given in column 1 below: those 
of the second specimen appear in column 2. 
I 2 
Dorso-ventral diameter at second septum.. 48mm. 56 mm. 
Lateral diameter at second septum....... 37mm. 44 mm. 
Dorso-ventral diameter at ninth septum.. 35 mm. 
Lateral diameter at ninth septum........ 30 mm. 
Mensth oiibody chamber arn yy) a) ian) 38mm. 47 mm. 
Dorso-ventral diameter of aperture... ... 37mm. 43 mm. 
Lateral diameter of aperture: ........... 22mm. 37 (?, broken) 
Average spacing of septa............... 4.5mm.1 6mm. 
As no shell is preserved, the character of the surface cannot be 
ascertained. The casts show a row of bead-like markings along the 
posterior margin of the chamber of habitation; this feature, however, 
is not uncommon in casts of other species of the genus. 
The first two septa posterior to the chamber of habitation are more 
closely set than is indicated in the figures given above: this is particu- 
larly well marked in the larger of the two specimens. The remaining 
septa are fairly evenly spaced. The forward arching of the septa is 
very slight. The siphuncle is small and marginal in position. 
Whiteaves describes three species of Poterioceras from the Galena- 
Trenton and Black River formations of Lake Winnipeg’—P. nobile, P. 
apertum, and P. gracile: the first of these species is much larger than our 
specimens and contracts much more rapidly towards the aperture; 
the second species is a less elongate form. With almost the same dorso- 
ventral diameter as our smaller specimen the height of the body 
chamber is only 31 mm. and the shell tapers more rapidly posteriorly. 
P. gracile could not possibly be mistaken for the present form, as it is 
extremely elongate with a chamber of habitation 115 mm. long and 
only 35 mm. wide. 
Named for the collector, J. B. Tyrrell, Esq., of Toronto, chief of the 
Hudson Bay Exploring Expedition of 1912. 
Locality—Lower rapids, Shamattawa river, Manitoba. 
Horizon—Ordovician. 
No. 320 S. Royal Ontario Museum of Palaeontology. 
1 Through a printer’s error or otherwise, this measurement is given as 3.5 mm. in 
the original preliminary description. 
2 Trans. Royal Soc., Can., Vol. VII, sect. IV, pp. 77 and 78, pl. XIV, figs. 204; pl. 
XV, fig. 1, 1889; Ibid., Vol. LX, sect. IV, pp. 87-88, pl. XI, figs. 2 and 3; pl. XII, figs. 4 
and 4a, 1891. Also Geol. Sur. Can., Palaeoz, Foss., Vol. III, pt. III, pp. 216-220. 
