THE DEVONIAN FOSSILS OF CANADA WEST. 347 
the inner surface of the shell by irregular rows of small tubercles. 
It is probable that when perfect the surface is always covered with 
small spines, as seen in the following figure. 
Fig. 120. 
Fig. 120. S. Ampla.—Dorsal view of a specimen covered with spines, from the Corniferous 
Limestone. The lower figure shows the area and foramen; the left hand figure, the longi- 
tudinal section. 
In the specimen above figured the spines are seated upon the crests 
of the radiating ridges. They are about two-thirds of a line in length, 
slightly curved, and appear to be tubular. They do not seem to have 
any connection with the pores of the shell, as these are situated, not 
on the radiating ridges, but in the grooves between them. In the 
interior of the ventral valve the muscular scars have very nearly the 
shape of those of S. inequistriata, except that they are proportionally 
broader. The divaricators are divided into four or five longitudinal 
concave lobes by as many obscure ridges. At their anterior margins the 
shell is thickened so as to make a sort of elevated border. The rostral 
septum is, in some specimens, rudimentary, in others, well developed. 
The whole of the internal surface appears to be covered with small 
tubercles. These leave punctures in the cast of the interior. 
I have only partly seen the interior of the dorsal valve. The diva- 
ricator processes resemble those of S. demissa. 
Affinities and variations.—This species has in general a semicircular 
or broad semioval contour, but it sometimes approaches the triangular 
form from being narrowed towards the front. The form of the cur- 
vature of the valves is subject to innumerable modifications ; the only 
constant curves being the general ones above stated. I think all the 
Specimens in our collection from the Oriskany Sandstone, Corniferous 
