330 THE DEVONIAN FOSSILS OF CANADA WEST. 
(OWT LY 
WM 
- a ae 
Fig. 103. Fig. 104. 
Fig. 103.—Strophomena inequistriata.—Conrad. Interior of ventral valve; 8.S..—the sides ; 
M.—the front margin; C.C.—the cardinal angles; the edge of the area from C to C is the 
hinge line ; A.—the flat space terminating the shell on the straight side is the area; B.— 
the beak; the small linear ridge beneath the beak is the deltidium; T.—teeth; R,—the 
rostral septum ; D.—the divaricator muscular scar or impression ; O.—the occlusor; V.— 
the vascular impressions. This figure is drawn as if the shell were flat in order to show 
all the parts more clearly. 
Fig. 104.—Strophomena demissa.—Conrad. Interior of dorsal valve. P.—the two divarica- 
tor processes or levers for opening the valves; O.—the occlusor muscular impressions or 
scars. 
this is absent. Valves articulated together at the hinge line or inner 
edge of the area, by teeth in the ventral valve, and sockets in the 
dorsal, the structure of which varies in different species. Surface 
ornamented with fine or coarse radiating strize or small ribs ; in a few 
species smooth. 
On the inner surface of the ventral valve there are two large pyri- 
form or subtriangular muscular scars or impressions, situated one on 
each side of the median line and in the upper half of the valve. 
These are the impressions of the DivARIcATOR MUSCLES or those whose 
function it was to open the valves. Between them there are two 
much smaller scars situated also, one on each side of the median line. 
These are the impressions of the OccLusor muscLEs, or those whose 
function it was to close the valves. On comparison it will be seen 
that the arrangement of the scars in the ventral valve is in a general 
way the same as in A¢hyris and Spirigera. It is nearly the same in 
Orthis, Chonetes, Producta, Atrypa, Spirifera, and in most other 
genera of Brachiopoda. 
In some species, but not in all, the cavity within the beak and umbo 
of the ventral valve is divided into two compartments, by a vertical 
