40 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
The convex surface of the platform is marked by deUcate concentric 
lines of growth, the concave surface by similar lines and by very fine 
radiating lines. 
All the muscles of the pedicle valve are attached to this concave 
surface; none is implanted directly on the valve. It is remarkable 
to find so delicate and fragile a platform carr\'ing all the strain of the 
adductor, diductor, and pedicle muscles. However, the impressions 
of the respective sets are so light that it is impossible to determine the 
points of attachment of some and the exact limits of others. The 
diductors appear to fill most of the cavity but the points of attachment 
of the pedicle muscles are unknown, except for a small scar on each 
side close to the hinge line, consequently the relative extent of the 
two sets is uncertain. The adductor scar is a narrow, tongue-shaped 
area lying in the median line on the posterior half of the platform. 
It is always quite distinct when the specimen is in good condition and 
is slightly variable in length. 
Considerable variation occurs in the shape of the spondylium. 
It may be deeply concave with sides almost parallel at the margins. 
A cross section of such a one made just anterior to the hinge would 
show an arc of 180°. A section along the median line would show a 
considerable concavity in the bottom of the plate between the anterior 
end and the tip of the beak. Such an individual is shown in figures 
42 and 43 on plate 8. Compare these with figures 37 and 38 on the 
same plate, in which the plate is much less curved in all directions. 
Figures 38 and 43 are drawn with the specimens held in the same 
position, so that any difference is due to the shape of the plate and not 
to a difference in viewpoint. The majority of the spondylia exam- 
ined are shaped more nearly like the one first described. 
Two accessory plates occur in the beak, one on either side of the 
spondylium. They extend from the outer surface of the spondylium 
to the inner surface of the shell and usually lie close to the hinge line. 
Whether they functioned as braces or were degenerate and function- 
less is not certain, but the latter appears to be a plausible explanation 
of their existence. They are variable with age. In the young they 
are relatively much more important than in the adult, where they 
may at times be reduced to the merest trace. In the specimen shown 
in figures 42 and 43 (pi. 8) they are more pronounced than usual. 
This feature is taken up in detail in the discussion of the possible 
line of descent of the type. 
