68 TRANSACTIONS OF THE [pees 9, 
of the pecten-shell and followed closely every groove and ridge 
of the latter. As the lime-salts, which constitute the shell-sub- 
stance, were secreted along the edge of the “ mantel” they must 
have been consolidated in the form of the mantel plications. 
In the figure I have drawn the pecten-shell with the Crepv- 
dula in its natural position and in natural size. The ridges of 
one-half of the scallop-shell are numbered from 1 to 9. 
The grooves of the pecten-shell are drawn black, and the 
grooves of the Crepidula are cross-striped. 
We see that the four entire grooves, respectively, ridges of 
Crepidula coincide in their proximal and distal points. 
The first ridge to the right (a) starts at ridge 7 of the scallop- 
shell and ends at the same ridge. Ridge b starts at ridge 6 and 
ends at same, and so forth. 
Ridge d is not quite complete, and from this point towards 
the left side we find no more ridges in the Crepidula-shell. I 
cannot give any explanation for this fact. 
It strikes us that the vertical projection of the ridges in 
Crepidula consists not in straight lines, but in curved ones. It 
seems as if the furrows of Crepidula would cross over the ridges 
of pecten. Yet this is not the case. . 
As the first explanatory point I have to mention, that the shell 
of Crepidula is intensely curved, whereas the pecten-shell is 
nearly flat in comparison. 
