142 THE DEVONIAN FOSSILS OF CANADA WEST. 
according to his intended meaning, and confined to such species as 
have the beak imperforate, and usually a mesial septum in the dorsal 
valve. For these the name Athyris is perfectly proper and involve 
no contradiction whatever. The type of this group would be Athyris 
tumida, as given by Davidson in his Introduction. 
2. D’Orbigny’s definition also literally, and it would include all 
the species with perforated beaks which have Spirigera concentriea 
for the type. The mesial septum in the dorsal valve in this genus is 
either rudimentary or entirely absent. 
3. the genus Athyris being limited as above, two sub-genera 
might be subtracted from it, that is to say, Merista—Suess, and 
Nucleospira—Hall. 
According to Professor Hall’s recent proposals, Spirigera must be 
suppressed, and Athyris made to take its place. This would leave 
the first of the above groups without a name, and thus his genus 
Meristella would be accommodated. 
The following figures represent some of the internal characters of 
the above mentioned genera : 
Fig. 48. Fig. 49. Fig. 50. 
Fig. 48.—Athyris tumida—Dalman.—Interior of ventral valve. D.—divaricator impressions 
O.—occlusor impressions. Z.—teeth. 
Fig. 49.—Interior of ventral valve of Athyris Clara —Billings. 
Fig. 50.—Athyris Clara, interior of dorsal valve. 
In the interior of the ventral valve of 4. tumida, Fig. 48, the two 
elongate oval scars which indicate the place of the attachment of the 
divaricator muscles or those whose function it was to open the valves, 
are situated side by side about the centre of the shell. Above, or 
partly between, is the small heart-shaped scar of the occlusor, the 
muscle that served to close the valves. Beneath the beak is seen the 
wide triangular foramen which, in consequence of the close incurvation 
of the beak is always completely closed. This foramen is a different 
