Permian Productinae and Strophalosiinae of W.A. 
49 
The interior of the brachial valve (20455) is geniculate, due to thickening 
of the lamellae in this region, a narrow groove (cincture) separating the 
visceral disc from the trail. In the visceral part the valve is almost divided 
by the median septum which continues forward for two-thirds of the length 
of the visceral disc; it is continued posteriorly as a trilobate cardinal process, 
at the base of which are deep sockets. The adductor muscles are below the 
sockets and set in a depression which gradually decreases anteriorly (this 
depression is bounded posteriorly by a ridge parallel to the hinge-line). The 
anterior adductors are elongate postero-anteriorly. The posterior adductors 
are elongate laterally and from their lateral extremities arise the brachial 
impressions. These continue the line of the ridge below the sockets until 
they almost reach the lateral border, they then follow the lateral and antero- 
lateral margins of the visceral disc and turn upwards parallel to the median 
septum. They cannot be traced in this specimen beyond the level of the 
end of the median septum. 
Dimensions of Shells. 
Height. 
Curvi- 
linear 
length of 
pedicle 
valve. 
Length 
of hinge- 
line. 
Max. 
width. 
Thick- 
ness. 
Holotype 
28-2 
56-0 
22-0 
27*8 
10*8 
Paratypes — 
20460 
19-2 + 
31-0-h 
23-7 
30*0 
20455 
24-3 
24-5 
29*3 
Brachial Valves 
29-3 
29-4 
35*7 
25-0 
25*24- 
28*1 
Combined Valves 
24-9 
43-0 
23*9 
28*5 
14*5 
29-0 
46-0 
25*9 
28*8 
14*4 
25*0 
42*0 
24-5 
28*7 
9*8 
As may be seen from these figures, the height and width of the valve 
are the same in full-grown specimens, but as the maximum width is situated 
in the posterior half of the shell, younger specimens are wider than long. 
The pedicle valve varies in shape from quadrangular to almost semi- 
circular and in convexity from weakly convex to hemispherical. As the shell 
becomes more strongly curved with age, the convexity is an indication of 
the age. There is, too, a difference in shape of some of the brachial valves 
due to the varying amount of thickening anteriorly; thus the value may be 
regularly concave or slightly geniculate. 
Owing to decortication of the shell surface, it is impossible to describe 
the tiue ornamentation of the shells. The lamellae are distinct on both valves 
and some specimens of the brachial valve show that these are crossea Dy 
very fine radial striae. A iew only of these are seen, but they are continuous 
from one lamella to the next. One pedicle valve has, also, a small patch of 
radial ornamentation preserved. It is ]>ossible, therefore, that, were the shell 
surface preserved, the shell could be described as radially striate; alterna- 
