THE DEVONIAN FOSSILS OF CANADA WEST. 339 
extending to the cardinal angles. The dorsal valve is usually not so 
much curved as the ventral, thus leaving a comparatively large space 
for the animal. 
Area of ventral valve from one-fourth of a line to one line in width, 
flat or concave, obliquely striated all except about one-tenth the 
length at each extremity, a wide shallow notch on the edge, in the 
middle of which is the foramen. Dorsal area about half-a-line wide 
and not so variable in its dimensions as is the ventral. 
A 
F i g. 114, 
Fig. 114.—A fragment of the ventral area natural size, shewing the foramen and the wide 
notch in the edge of the area, 
Foramen small, linear, closed, usually about one-fourth of a line 
wide, sometimes less. Teeth rudimentary, and situated one on each 
side of the foramen on the edge of the area. Cavity of the beak 
divided into two compartments by a rather strong rostral septum. 
In the interior of the ventral valve the divaricator scars are large, 
sub-pyriform, and one-third the length of the whole shell. The 
occlusors are ovate, half the length of divacirators, often with the 
surface covered with minute corrugated wrinkles like the scars of 
some species of Producta. The vascular impressions are well marked 
on some of the casts of the interior, but vary in the number of the 
branches, usually from three to five in the width of one line at the 
margin. In thin shelled individuals they are not seen at all. Inte- 
rior of dorsal valve not observed. 
Surface very variously striated. In some the strie alternate in size, 
there being one set of fine sharply elevated lines distant from half a 
line to one line from each other, the intervening spaces flat and with 
from three to seven finer strice just visible to the naked eye ; in others 
the intervening spaces are concave. In many the principal striz 
become coarser and closer together until the whole surface is covered 
with strong angular bifurcating ridges from one-fourth of a line to 
half a line in width. In very well preserved specimens of these 
latter, the coarse ridges are seen to be themselves ornamented with 
the fine longitudinal striz. In all cases the whole surface when per- 
fectly preserved, is beautifully cancellated by minute crowded concen- 
tric striz. 
Affinities of this species—This species belongs to a type which 
