42 The American Geologist. J"'>'» i^^^. 
4. Artificial cast from a mold of the exterior. The reconstructed 
portion was broken away before fossilization. 
5. Internal natural cast of the same specimen. 
6. Dorsal view of the same cast. 
7. Outlines of figs. 4 and 5 in normal relation to show the shell 
thickness on the dorsum. 
8. Right valve x2 reconstructed. 
0. Left valve x2 reconstructed. 
Specimens figured are all from the Galena (Trenton) stage, zone no. 
2. Figures i, 2 and 3 are of specimens from Minneapolis, Minn. : 
fig. 4 — 7 are from Argyle, Wisconsin. 
The consolidation of the sediment would presumably be 
by vertical compression, as is evidenced in the distortion of M. 
plana; those casts which lie horizontally being, flat, (fig. i.) 
upright ones being convex (fig. 2.) and rarely one standing on 
end is then shortened (fig. t,.). Part of this distortion is how- 
ever due to compression after the matrix w^s consolidated. 
There should be distinguished therefore (a) ' compression 
during solidification of the matrix, affecting M. plana but not 
other pelecypods, the shells of which were yet extant, and (b) 
compression since the solidification and after the removal of 
the shell substance of other molluscs and from this all have 
suffered distortion more or less, according to locally differ- 
ent conditions. 
A specific reconstruction from a series of these fossils to 
determine both the original form, bv estimating the amount of 
distortion . from vertical compression of the matrix, and also 
the thickness of the shell as evidenced by the sometimes 
strongly marked anterior muscle scar and pallial line, would 
seem to indicate that the original shell or its fossil cast if pre- 
served like associated ones, would be verv different from that 
which M. plana H. was supposed to be. Fortunately speci- 
mens have been found which arc preserved like other pele- 
cypods and although the}' at first appear to be a different spe- 
cies and even genus they are simply the normal of the same. 
In one of the shelly rock laminse filled with casts and molds 
of all shells at Alinneapolis, a cast of J\I. plana H. has been de- 
tected Avhich seems to have escaped the total compression ow- 
ing to protection of surrounding shells. At Argyle and 
Dodgeville, Wisconsin, I have found also several normal 
molds and casts, one of which is here figured (figs. 4, 5, 6.) 
and, near these, other specimens which are compressed in 
