THE DEVONIAN FOSSILS OF CANADA WEST. ZH: 
others it is either a little greater or a little less. Greatest width, 
about the middle, or a little in front of the middle, at which point the 
rhomboidal specimens are angular, but in the more oval forms, gently 
convex. 
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Fig. 33. Fig. 34, 
Fig. 38. Athyris Maia.—Dorsal aspect. Fig. 34.—The same ; ventral aspect. 
The following characters may be more particularly noted : 
The umbo of the veutral valve is rather large and prominent, the 
beak well defined, strongly incurved, but in general not im contact 
with the umbo of the dorsal valve ; a short false area beneath the beak. 
The mesial sinus is shallow, evenly rounded in the bottom, or some- 
times with an indistinct fold along the middle. Its width at the front 
margin is somewhat variable, but is usually about five lines, and it 
becomes gradually narrower and shallower upwards, but is more or 
less distinctly visible quite to the beak. On a side view, the outline of 
this valve presents a continuous curve, most abrupt in the upper half, 
the greatest elevation being at about one-third the distance from the 
beak to the extremity of the linguiform extension in front. 
The dorsal valve has a smooth, rounded fold, extending almost to 
the beak, but is otherwise pretty evenly convex. It appears to pos- 
sess a straight hinge-line, the length of which is greater than half 
the whole width of the shell; the beak small, not much incurved. 
The greatest convexity of this valve is about the middle, and on a 
side view the outline, in consequence of the elevation of the mesial 
fold, continues at the same height, and somewhat parallel to the 
lateral margin until it reaches the front. 
The linguiform projection is sometimes considerably extended, 
- and the shell has then a rounded, rhomboidal form, but in other 
specimens this part of the shell is truncated, and a fifth side, situated 
in the front margin, is thus formed. 
