THE DEVONIAN FOSSILS OF CANADA WEST. 331 
ridge or septum. This I propose to call the Rosrran Seprum. It 
varies greatly in size, and is often absent altogether. It cannot there- 
fore be regarded as an organ of generic importance. 
In the interior of the dorsal valve there are four small scars arranged 
in two pairs, one pair on each side of the median line. These are the 
Occtusors corresponding to those of the ventral valve. In this 
valve the divaricators were attached to two small processes situated close 
to the hinge-line. (See fig. 104, P.) These are notched at their extre- 
mities, and grooved on the outside, or side next the area. These I 
propose to call the Divaricaror Processes ; their function was to 
open or divaricate the valves; the mechanical principal upon which 
they operated was simply that of the lever. This will be more clearly 
understood by consulting Fig. 105. . 
Fig. 105. 
Fig. 105.—A longitudinal section through both valves of a Strophomena from the beak to 
the front margin ; the dorsal valve uppermost. M.—the front margin; A.—the area of 
the ventral valve; S.—the socket in the dorsal valve for the reception of the teeth of the 
ventral valve; P.—the Divaricator Process (or lever); D.—the Divaricator Muscle; O0.— 
the Occlusor. It is evident that by the contraction of the Divaricator muscle the extremity 
of the process P must be drawn towards the point D, and thus the dorsal valve must turn 
on the hinge at S, (as a door turns on its hinges.) By this movement of course the valves 
were separated at the front margin M. By the contraction of the Occlusor O, the valves 
were drawn together. It appears that in most of the Pal@ozoic genera of Brachiopoda 
the muscular apparatus consisted of these two sets of muscles, but a little modified in 
different groups. 
In Strophomena the form of the scars and their distinctness varies 
to some extent in different species, but their arrangement is in a 
general way the same in all. . 
In addition to the muscular scars, the inner surface of many species 
exhibits numerous radiating branching channels, usually most distinct 
near the margin. These are the impressions of the vessels of the 
vascular system. 
We shall now notice more particularly some of the variations 
exhibited by the parts above mentioned in connection with the 
following proposed genus. 
