AMERICAN NATURALIST 
VoL AA VIIL November, 1894. 335 
THE MECHANICAL CAUSE OF FOLDS IN THE APER- 
TURE OF THE SHELL OF GASTEROPODA. 
By WM. H. Darr. 
. The folds which are frequently present on the columella 
and the lip of the aperture of the shells of Gasteropoda, may, 
I think, be traced to a mechanical cause. In considering the 
dynamie relations of the animal to its shell we may obtain 
satisfaction on this point. In the fusiform rachiglossa an 
anatomical difference exists to which I believe attention has 
not hitherto been called. Indeed, unless the principles of 
dynamic evolution are granted it is a difference which would 
appear to have little or no significance. These principles, 
however, afford a key which seems to unlock this and many 
other mysteries. In the plicate forms of this sort the adductor 
muscle, which in all gastropods is attached to the columella 
at a certain distance within the aperture, is attached deeper 
within the shell than in non-plicate forms. The point of attach- 
ment may be an entire turn, or even more, behind the aper- 
ture, while in short globose few-whorled shells and in the non- 
plicate forms it is, as a general rule, little more than half a 
turn within the aperture. 
‘Adapted from the Transactions of the Wagner Free Institute of Science, Phila- 
delphia, Vol. III, 1890, p. 58. 
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