342 THE DEVONIAN FOSSILS OF CANADA WEST. 
The most common size is from one inch to one inch and a half in 
width, but very small specimens of from four to twelve lines are often 
found. 
The ventral valve is in general rather strongly convex, uniformly 
arched from beak to front, sometimes a little flattened in the central 
region; the umbo small, rounded but prominent, overhanging the 
area, the shell on each side depressed or subconcave towards the 
cardinal angles ; in some a broad obscure carination extends from the 
umbo along the middle to the front, with. an obscure longitudinal 
depression on each side. 
Dorsal valve moderately concave, usually with a shallow mesial 
sinus commencing in a point at the beak and growing wider towards 
the front. 
Area of ventral valve in some specimens broad and somewhat fiat : 
usually narrow ; often concave beneath and on each side of the beak, 
either striated the whole length, or with a very small portion at the 
cardinal angles smooth. Dorsal area not so variable as the ventral ; 
the two areas inclined to each other at an angle which varies from less 
to greater than a right angle, according to the degree of curvature of 
the beak of the ventral valve. 
No foramen ; a smooth triangular space beneath the beak on the 
area of the ventral valve. 
Surface with from ten to fifteen coarse angular ridges on the umbo 
of the ventral valve which bifurcate several times, and become smaller 
towards the front margin. In some small specimens the ribs do not 
bifurcate. 
In the interior of the ventral valve the muscular scars are of the 
same type as those of S. inequistriata, but the occlusors are proportion- 
ally nearer the beak. In the dorsal valve the occlusors are situated 
in the upper one fourth of the length of the shell; they are divided 
by a median ridge which sometimes is much elevated about the middle 
of the shell. There are usually two or three large tubercles or short 
curved ridges just in front of the impressions. In thick shelled 
specimens, the scars, median ridge and tubercles form a group 
occupying an oval space which extends nearly half the length, and is 
narrowed to a point below. Insome there is a large space around the 
muscular area covered with small tubercles; in others this space is 
smooth. The vascular inpressions are only well marked near the 
margin. 
