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 

 branchiae be single or double. 



PHOLAS DACTYLUS. (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 branchiae in the Bivalves. 



Dr. Sharpey's original views on the permeability of the 

 gill-laminae in the Acephala, have, after a lapse of more than 

 twenty years, again been prominently 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. vii. p. 108 of ' London'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 this condition has been withdrawn in a paper by Mr.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, 



2 L 



