28) THE DEVONIAN FOSSILS OF CANADA WEST. 
Figs 42. 
Fig. 39. Athyris wnisulcata —Ventral view. Fig. 40.—The sae: dorsal aspect. 
Fig. 41.—View of front margin. Fig. 42,—Side view.* 
Description.—Shell quadrilateral or sub-triangular; greatest width 
near the front margin ; sides nearly straight from the beak to the line 
of the greatest width, where there is a prominent angle on each side 
of the shell, thence converging and nearly straight to the middle of 
the front margin. Dorsal! valve with a mesial foid which occupies the 
whole of the shell except a small portion on each side in the upper 
half; the slope from the ridge of the fold to the sides usually gently 
concave. In some specimens a single strongly impressed groove along 
the ridge of the mesial fold. The ventral valve has a deep mesial 
sinus equal to its whole width; a small strip of the margin between 
the beaks and the point of the greatest width mflected at a right angle 
or thereabout towards the dorsal valve. The beak is incurved over 
the umbo of the dorsal valve, but its tip not quite in contact with the 
surface of the dorsal shell. The surface is nearly smooth, a few ob- 
secure concentric undulations, and, in some specimens, indications of 
fine radiating striz visible. 
Length of large specimen, nine lines ; width, eleven lines. 
This species varies greatly in form. Some have the front margim 
nearly straight, and are thus almost triangular. Others are quadran-. 
gular or rhomboidal from the great projection in the middle of the 
front margin. The sinus of the dorsal valve is sometimes so shallow 
* hese figures are not very good, but as they will serve to give an approximate idea of the - 
form of one of the varieties, I have thought it best to use them. 
