CANADIAN FOSSILS. IT 



Prof. Emmons' O. complanata, as figured by Hall (Palaeontology of 

 New York, vol. i., p. 11), is nearest in size, but is still greatly smaller, 

 and has even more whorls in a shell of half the diameter. O. levata 

 is a minute species. 



At first sight this shell, like the Machrca (plate 1), appears to be 

 reversed or left-handed, the flatter side apparently being the spire, 

 as in 'many Euomphali, and the concave one the wide umbilicus 

 (Euomjihalus calyx, of the mountain limestone, in form greatly 

 resembles fig. 3). But when comparison is made with such forms 

 as that last described — Hdicotoma {H. uniangulata, for instance) — the 

 real affinity is manifest ; although the evolution of the whorls is 

 extreme in the present case, the direction of the lines of growth is 

 decidedly similar in the two genera. 



Diameter, about one inch and a half, and height of outer whorl 

 about six lines. 



General form thick-discoidal, the upper side shewing five or six 

 whorls, rather slowly increasing in size, steeply concave from the 

 outer margin, so as to form a hemispheric cup. The whorls have 

 each a sharp keel outside, which rises prominently above the general 

 concave surface (in the cast making deep impressed furrows, fig. 1), 

 and the lines of growth are turned considerably backward, from the 

 suture to the prominent upper keel, a. On the outer vertical face 

 they make a bold curve forwards, retreating again, fig. 4, to the 

 lower angle, b. 



The umbilical face, fig. 2, makes nearly a right angle with the 

 vertical outer margin. We are unacquainted with the course of the 

 lines of growth over this. The mouth, fig. 1 ah, is trapezoidal, the 

 inner side, c, vertical like the outer, and parallel to it, but not above 

 half the length. Fig. 1 being reversed, the lower or square angle of 

 the mouth is indicated at c. The upper side, fig. 3 ad, takes of course 

 the same slope as the spire generally, and forms with the outer face 

 an angle of less than 45°. 



The species has been quoted by me from the Lower Silurian rocks 

 of N. W. Sutherlandshire, where it occurs in a thick cherty limestone 

 overlying quartz rock.* A fresh comparison shews only very trifling 

 differences. The English fossil is not quite so large, and has rathet 

 more whorls in a given space ; but these proportions vary, and the 

 form is exact. The O. (Madurea) sordlda appears to accompany it 

 in Sutherlandshire as it does in Canada and New York. 



» See Sir R. I. Murchison's Memoir on the Rocks of the N. W. of Scotland. Quar- 

 terly Geological Journal, vol. 14, ined. 



B 



