THE DEVONIAN FOSSILS OF CANADA WEST. 275 
its beak strongly incurved. Length from one to two inches; width 
equal to, a little less or a little greater than the length. The ordinary 
size is about one inch and a half in length. 
The ventral valve is strongly and smoothly convex, the outline evenly 
arched from beak to front, more abruptly curved above than below, 
the umbo prominent, the beak rather small, neatly rounded at the 
sides, and closely incurved. The linguiform projection in the middle 
of the front margin, is often a simple extension of a portion of the 
shell, without a sinus, but occasionally there is either a short, shallow 
depression, or a narrow rounded mesial fold, which seldom, however, 
extends towards the beak more than three or four lines. The upper 
half of the dorsal valve is sometimes evenly convex, but in general an 
indistinct, more or less broadly rounded carination, can be traced from 
the umbo along the middle to the front, where it becomes abruptly 
elevated into a short, prominent, rounded fold, which extends into the 
linguiform projection. On each side of the median line, this valve is 
gently convex, and often exhibits a rather flat slope to the lateral 
margins. The beak is strongly incurved, and appears to be deeply 
buried in the cavity beneath the umbo of the ventral valve. 
Ifa line be drawn across the shell at mid-length, and another at 
one-fourth the length from the front, the greatest width will be found 
to range between the two. Many of the specimens are obtusely 
angular at the sides, and in such the margins above and below the 
angles are somewhat straight, the upper two sides converging to the 
beak, and the lower two to the linguiform extension in front, giving 
to the shell a rhomboidal instead of an ovate outline. 
At first sight, the surface appears to be smooth, with a few con- 
centric squamose lines of growth. On closer examination, numerous 
indistinct, radiating lines, may be seen. Of these, there are from two 
to four in the width of one line, and they sometimes appear to lie 
beneath the surface of the shell. In very perfect specimens, the surface 
exhibits fine concentric striee, from ten to fifteen in the width of one 
line, and these are most distinct towards the front margin. 
Beneath the beak of the detached ventral valves, there is a wide, 
triangular foramen, not visible when the valves are united. The inside 
of the beak is entirely hollowed out into a deep pit or channel, which 
opens directly into the cavity of the shell. The impressions of the 
divaricator muscles occupy part of a sub-triangular space, the upper 
angle of which is situated just where the excavation beneath the beak 
