268 THE DEVONIAN FOSSILS OF CANADA WEST. 
Description.—Sub-orbicular or sub-quadrate ; length about eight- 
ninths of the width; greatest width, usually a little in front of the 
middle; length of hinge line, one half to two-thirds the width of the 
shell; cardinal extremities rounded; sides in most specimens some- 
what straight, often sufficiently curved to give a circular aspect to the 
shell ; front angles obtusely rounded ; front margin in general broadly 
convex, sometimes with a small space in the middle nearly straight. 
Dorsal valve of a medium convexity, most elevated about the middle ; 
the outline forming a uniform arch from the depressed beak to the 
front margin; the slope from the umbo towards the cardinal angles, 
gently concave; sometimes a barely perceptible mesial depression, 
commencing ‘in a point at the beak, and becoming obsolete at one half 
or two-thirds the length; area small, lying in the plane of the lateral 
margins; beak minute, forming a small triangular projection, rising 
scarcely one-fourth of a line above the edge of the area, the point not 
incurved over, but situated in the plane of the area. Ventral valve 
moderately convex, most elevated at between one-fourth and one-third 
the length from the beak, thence descending with a somewhat flat or 
gently convex slope, to the front and sides, and with a more sudden 
and somewhat concave slope to the hinge line and cardinal angles ; the 
umbo small, prominent, neatly defined, terminating in a small rounded 
beak, which is incurved so as to overhang the edge of the area, either 
not at all or scarcely one-tenth of a line; area triangular, about one- 
fourth larger than that of the dorsal valve, forming an angle of about 
105° with the plane of the lateral margin. The foramen not observed, 
but appears to be wider than high. On looking at the dorsal valve in 
a direction perpendicular to the plane of the shell, the small rounded 
umbo of the ventral valve can be seen rising about one-third of a line: 
above the dorsal beak. . 
Surface with small sub-angular radiating ridges, of nearly a uniform 
size, from eight to ten in the width of three lines, increasing by bifur- 
cation, strongly curved outwards to the upper part of the sides and 
cardinal angles, the intervening grooves sub-angular in the bottom, and 
equal to the ridges in width. In very perfect specimens, very fine 
concentric sub-lamellar concentric striz are visible, seven or eight to 
one line. In certain conditions of preservation also, the radiating 
ridges are seen to be sub-tubular, and exhibit numerous small oval 
or circular openings on their edges, each about the eighth or tenth of 
a line in width, and from one-fourth to two-thirds of a line distant 
from each other. 
