THE DEVONIAN FOSSILS OF CANADA WEST. 337 
cators are situated one on each side, and the occlusors (seldom well 
defined) between them. The form of these scars appears to be at 
first sight somewhat different from that of S. inequistriata but on 
a little examination it will be seen that the general arrangement is the 
‘same and the form only specifically different. On each side of the 
foramen is a single short tooth. 
In the interior of the dorsal valve the divaricator processes consist 
of two short elevated ridges terminating abruptly just over the area, 
their extremities not elevated, and free as they are in S. demissa. 
They are separated in some specimens (but not in all) by a deep oval 
pit. On each side is seen a small oblique socket or pit for the recep- 
tion of the tooth of the opposite valve. Just in front of the divari- 
cator ridges are the two small scars of the occlusor muscles, each 
scar divided into two by an oblique ridge not often well developed, 
but distinctly seen in a beautiful specimen now before me. These 
scars are small, each pair occupying a space only one line in length 
and breadth in a specimen one inch wide. The two pairs of scars 
are separated by a low mesial ridge, which in some specimens becomes 
a thin elevated septum towards the front of the shell. The vascular 
impressions are only well marked round the margin. 
Width from one-inch to one-inch and a-half; length about one- 
third less than the width. 
Specimens two inches wide sometimes occur. 
Affinities.—This wonderful species has no near relatives in the 
Devonian rocks. By the form and structure of its foramen, divari- 
cator processes and muscular impressions, it is clearly a Lower Silurian 
type belonging to the group, which includes S. alternata, and its 
varieties S. deltoidea and S. tenuistriata (Pal., N. Y., Vol. 1). It 
commenced its existence just at the close of the Lower Silurian 
period; or perhaps a little earlier, and lived on, with scarcely any 
change through the immeasurable ages of the Middle and Upper 
Silurian and Devonian, and even until the Carboniferous was well 
advanced. 
Locality and formation.—Occurs at nearly all the localities of the 
Oriskany Sandstone, Corniferous Limestone and Hamilton group in 
Canada West. Also in all the older formations down to the top of 
the Hudson River group. 
Collectors.—A. Murray, E. Billings, J. De Cew, E. De Cew, Judge 
Wells, Chatham, Wm. Saunders, London. 
