PHOLAS. 173 
vestigation of fourteen weeks, my doubts were dispelled, and 
I stated personally to Dr. Battersby, that after a careful 
review of all the evidences that presented themselves, I re- 
verted to and relied on my original determination of identity 
of the two forms of Pholadidea papyracea. 
This change of opimion arose from the observation that in 
the adult Pholadidea papyracea, the mottled appearance of the 
belly, so dissimilar to that of the form Pholas lamellata, was 
due to the extension of the reproductive membranous organs 
of the ovarium occupying the space usually appropriated to 
the foot, which I found had disappeared. This anomalous 
appearance excited my attention, and the reflection that with 
nearly absolute ceteris paribus in the generalities of all the 
Pholades, there was no substantial reason why one species 
should always be deprived of the foot, when all the others 
possessed that appendage, and as I had come to the conclusion 
that it was the borg instrument, I felt assured that this 
anomaly was only an apparent one, dependent on certain 
conditions connected with the growth of the animal. And as 
the very large anterior gape in all the Pholades is the site of 
the powerful foot, and is never closed up during their exist- 
ence, except in this species, I became fully convinced that 
the foot,—having finally performed its terebrating functions, 
the animal consequently having arrived at full growth (the 
test of which is the doming and formation of the caliciform 
incipient tubing, which is in Pholadidea papyracea the last 
vestige of the protecting tubes of the Teredinide) —had 
become absorbed, on the well-known principle, that an organ 
from want of use is often, especially in the lower animals, 
followed by its total disappearance. 
I have already shown that the great variation in colour and 
markings between the adult Pholadidea papyracea and the 
young shell styled Pholas lamellata is the effect of generative 
influences, and that its conspicuous foot, when it arrives at 
full growth, which is testified by its becoming completely 
domed, is depauperated and finely obliterated. These two 
great and principal variations of aspect between the two forms 
of Pholadidea papyracea, resulting from states of transition, 
