Hogg, on the Lingual Membrane of Mollusca. 95 



pliopoda, Pteropoda, Gasteropoda, &c. In a second divi- 

 sion he made the respiratory organs a foundation for a 

 systemic arrangement ; but this has proved unsatisfactory, 

 for, although in most animals respiration appears to be 

 indispensable to life, special organs are by no means always 

 and absolutely necessary for the purpose. Thus, in some 

 vertebrates are found both lungs and gills, which, according 

 to J. Midler, are not homologous. They sometimes occur 

 together in the same animal, but do not exactly perform the 

 same function ; as we noticed in the case of the tadpole 

 described by my friend Mr. Whitney in a valuable paper 

 published in our ' Transactions.' Many of the mollusca, as 

 Cyclostoma, Neritina, and Litorina, are furnished with gills ; 

 nevertheless, they live frequently on land and breathe air. 

 Have they, like the land-crab, the power of keeping their 

 gills moist? Again, in those species unprovided with a 

 shell respiration in many individuals takes place almost 

 entirely through the skin ; when, however, a shelly covering 

 is fully developed, a respiratory organ of some sort is ne- 

 cessary. In short, it is generally admitted that neither 

 the respiratory nor the locomotive organs offer reliable cha- 

 racters for a primary division. 



The operculum is said by some authors to answer to the 

 second hard covering of the bivalves. Loven regarded this 

 appendage as homologous with the byssus, but this has 

 been shown to be erroneous, since a byssus is found in 

 some few univalves — the Cyclostoma suspenmm, Swanston, 

 Planaxis, Macdonald, Rissoaparva, Gray, «&c. The byssus of 

 Acaphale is corneous ; a calcareous plate forms a plug in 

 Anomia, and a pedicle in Tercbratula, which is looked upon 

 as " a secretion of the ventral face of the foot." Later 

 investigations seem to point to the conclusion that all parts 

 of the skin of mollusca can secrete shell, and probably the 

 same remark applies to the operculum. 



Some fewyears have now elapsed since two or three scattered 

 papers in the scientific periodicals of the day announced a 

 new classification of the mollusca, founded on the arrange- 

 ment of the teeth on the lingual membranes. Gray in this 

 country, and Troschel in Germany, appear to be the most 

 earnestly devoted to the object of carrying out in a syste- 

 matic manner this scheme of classification. The only paper, 

 however, on the subject, one which is likely to have fallen under 

 the notice of every Fellow of this Society, is from the pen 

 of Dr. Gray, published in Vol. I, n. s., 1853, p. 170, " On 

 the Teeth on the Tongues of Mollusca." I must particularly 

 refer you to this paper, as it offers a somewhat comprehensive 



