THE DEVONIAN FOSSILS OF CANADA WEST. 335 
there is every shade of gradation from an aperture two lines wide 
down to nothing. I hold, therefore, that the size of the foramen is 
too variable to be of value as a generic character. 
The same gradation occurs also in the extent to which the hinge 
line is crenulated. 
1.—Lower Situri1an.—Most of the species with the hinge-line 
and teeth smooth. 
2.—Mippie anp Upper Siturtan.—Most of the species with 
the teeth or a small portion of the hinge-line next the foramen 
striated. Ex. SS. Leda. S. Philomela. 8S. euglypha, &c. 
3.—Devonian.—Most of the species with a large portion or 
nearly the whole of the hinge-line striated. 
The striation of the area appears to have kept pace with the 
diminution of the foramen; the one gradually increasing from the 
Silurian upwards to the Devonian and the other as gradually diminish- 
ing. 
The striated hinge-line and area is not peculiar to Strophomena. 
Leptena transversalis and Chonetes hemispherica exhibit the same 
character, although most other species of these two genera do not. 
For the above reasons and also because there is no difference in the 
form of the shell, I hold that the genus Strophodonta is quite super- 
fluous. 
Number of species of Strophomena. 
On examining the various Reports of the Geological Surveys of 
the neighbouring States, I find that seveNTY-THREE species have been 
named as occurring in the Upper Silurian and Devonian Formations 
of these countries. According to my view, this number must be 
greatly reduced. I do not think there can be more than twelve or 
fifteen. In Canada West I can only recognize nine species in the 
Oriskany Sandstone, Corniferous Limestone and Hamilton group, and 
three of these, S. magnifica, S. magniventra and S. Pattersoni, may 
be only varieties, the first two of S. perplana and the last of S, 
inequistriata. 
