PHOLAS. l/ a 



vestigatiou 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 opinion arose from the observation that in 

 the adult Pholadidea papyracea, the mottled appearance of the 

 belly, so dissimilar to that of the form Pholas lameUata, 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 cceteris paribiis 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 boring 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 Teredinida;} - - 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 lameUata 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, 



