334 THE DEVONIAN FOSSILS OF CANADA WEST. 
Hh 
Fig. 108, ; Fig. 109. Fig. 110. 
Fig. 108.—Represents the muscular scars in a specimen of a species closely allied to, if not 
identical with S.alternata. It is from the Black River Limestone, Pallideau Islands 
Lake Huron, The scars are deep, and well defined. O.—the occlusors. D.—the divari-e 
cators. T.—the teeth. 
Fig. 109.—S. alternata. Hudson River group. The scars not well defined. 
Fig. 110.—S. Philomela. Middle Silurian. The scars welldefined. R.—the rostral septum, 
rudimentary. ©O.—occlusors. D.—Divaricators. 
When these are compared with Fig. 103, it will be seen that, 
although there is some difference in form, the arrangement of the 
muscular apparatus is the same, 7. e., the divaricators outside, and the 
occlusors between them. 
The same rule holds good with respect to the occlusors. In all the 
species (in which they have been observed) they are arranged in 
two pairs, one pair on each side the median line, and yet they differ 
in form according to the species. Even in different individuals im the 
same species they differ. Thus Fig. 107 differs from 104. Both of 
the figures differ from that given by Davidson in the Geologist, Vol. 
2., pl. 4, fig. 15, which was drawn from a specimen procured from 
Prof. Hall, and all of them differ from a specimen in my possession— 
from the Hamilton Shales of New York. 
With respect to the foramen, the specimens in our collection, and 
the figures given by various authors, show that there has been a 
gradual change in the size of the orifice. 
1.—Sinturian. Most of the species with the foramen large, its 
width greater than the height. Ex.--S. alternata, S. filitexta, 
S. planoconvexa, &e. 
2—Devonian. Most of the species with the foramen very narrow, 
Sometimes reduced to a mere line across the area of the ventral 
valve, and insome entirely absent. Ex.—WS. inequistriata, S. ampla, 
S. demissa. 
In comparing the fine series in our collection (embracing species 
from every formation, from rocks holding Primordial Trilobites up to 
the Corniferous), and also the figures given by Barrande, De Verneuil, 
Davidson, Hall, and others, it is clear that in the size of the foramen 
