APPENDIX. 513 
cover each other so closely as to be mistaken for one; but 
this is not always the case. It is difficult to separate the two 
plates, even when floated; we have, nevertheless, often suc- 
ceeded, and have a preparation which is indisputably decisive 
of their duplex character. There is one thick, rather narrow, 
elongated branchia, and a rudimentary one on each side of 
the body, and not two entire plates, as mentioned in former 
notes. ‘The green liver is conterminous with the pale red- 
brown ovarium, which at this period is filled with ova. 
It appears to be a non sequitur that a single branchia on 
each side should be accompanied by a corresponding single 
palpum. We apprehend all bivalves must have two palpi on 
each side, as purveyors of aliment to the mouth, whether the 
branchiz be single or double. 
PuHo.as pactyLus.—(P. 192.) 
Since the letter here printed was written, several papers have 
appeared in the ‘ Annals,’ which largely support, and partly im- 
pugn, our theory of the action of the branchize in the Bivalves. 
Dr. Sharpey’s origimal views on the permeability of the 
gill-laminze in the Acephala, have, after a lapse of more than 
twenty years, again been promimently brought forward. It 
was our lot to become fully cognizant of his discovery in 
1834, that is, soon after it was promulgated, as it appeared in 
vol. vil. p. 108 of ‘ Loudon’s Magazine of Natural History,’ 
to which we subscribed, and it has, as late as 1850, again 
been mentioned in Dr. George Johnston’s excellent volume, 
entitled ‘An Introduction to Conchology’; but considering 
Dr. Sharpey’s views erroneous, and physiologically impossible, 
we dismissed them from our thoughts. 
Messrs. Alder and Hancock have also announced an essen- 
tially similar scheme, as based on original observations ; but 
this condition has been withdrawn in a paper by Mr. Alder in 
the ‘ Annals of Natural History,’ vol. xiv. p. 131, N.S. 
And lastly, Dr. Thomas Williams, in a series of memoirs 
published in 1854 in the above work, on the aquatic respira- 
tion of the Invertebrate animals,—we particularly allude to 
those on the Mollusca,—has adopted Dr. Sharpey’s theory, 
21 
