54 GEOLOGICAL AND NATURAL HISTORY SURVEY OF CANADA. 



gently curved. Sulcus deep, well defined, situated just within the pos- 

 terior half, and extending from the dorsal margin about half way across 

 the bod}^ of the valve. Just below the sulcus the surface is somewhat 

 elevated, and about midway of its length, on the posterior side, is a 

 low rounded prominence which projects slightly into the sulcus. The 

 marginal area is strongly convex, especially in the ventral region, be- 

 ing there also terminated by a narrow but sharply reflexed border. 

 As the. dorsal angles at each end are approached, this border is gradually 

 lost in the general elevation of the area. On the interior, the marginal 

 hollow does not extend to the dorsal angles, but reaches up on each 

 side only about half the distance to the angles. 



Greatest length of valve, including marginal area, 2.2 mm.; length 

 of body of valve, l.Gmm.; width of entire valve, 1.22 mm.: width of 

 body of valve, 0.94 mm. ; convexity of same, 0.18 mm. 



This species is represented by two valves from Stony Mountain, one 

 collected by Dr. Ells in 1875, and showing the exterior, the other by 

 Mr. Weston in 1884, showing the interior. They resemble one of the 

 Birdseye limestone species more than any of the others known to me. 

 Prof. Jones suggests that it is comparable with M. strangulata, Salter, 

 sp. 



Strepula quadrilirata, Hall and Whitfield. 



PI. IX., fig. 12. 



Beyrichia quadrilirata^ Hall and Whitfield. 1875. Pal. Ohio, vol. 2, p. 105, pL 

 4, figs. 6-7. 



Of several hundred valves of this species examined by me none are 

 as quadrangular in outline as that shown in the figure given by the 

 authors of the species. Indeed, they are remarkably constant in their 

 form, deviating but little in that respect from the example now figured 

 on plate IX. These authors, it appears, failed also to notice the five 

 pits along the anterior half of the free margin, which is a very charac- 

 teristic feature of the species. The vertical plates which separate the 

 pits often project beyond the depressed border of the valve, appearing 

 in a view of the inner side like so many small spines. 



A comparison between Hall and Whitfield's fig. 6 and fig. 12 on 

 plate IX. of the present report shows other differences. In their 

 figures, the depressed outer or contact margin is much narrower, and 

 does not appear as overhung by the semicircular marginal ridge, nor 

 is this ridge represented on the ventral side at all. Their view of a 

 ^' basal profile outline" fits very well to their fig. 6, but is it true to 

 nature ? 



