94 Hogg, on the Lingual Membrane of Mollusca. 



soft parts of the mollusc, taken without reference to the shell, 

 offers an extremely slight and variable criterion of specific 

 difference." Dr. Gray asserts " that no sj)ecies of gasteropo- 

 dous mollusca can be properly placed in a system unless we 

 are enabled to examine the animal, the shell, the operculum, 

 and the structure of the tongue." The shelly covering is a most 

 ■ essential part of a very large number ; its structure is hard and 

 dense, and it is, so to speak, the skeleton placed outside instead 

 of within the animal. Or it may be regarded as a pseudo-skele- 

 ton, serving, not only to protect the soft parts, but also to keep 

 the whole fabric together, as the internal bony skeleton does 

 the fleshy parts of vertebrata. There is, it should be observed, 

 an equally intimate connection between the shell and soft parts, 

 which is only dissolved by death. The shell, therefore, being 

 the more permanent of the structures of a very large number 

 of mollusca, it is but natural to expect that it should remain, 

 as it, in fact, always seems to be, the most reliable means of 

 classification. 



The forms of shells are not only more permanent, but are 

 capable of reproduction without modification. The oldest 

 geological shells are indistinguishable from existing species. 

 " A large proportion of the fossil shells found in the lowest 

 of the Pliocene strata (coralline crag) are precisely similar in 

 every respect to the recent shells of species which still sur- 

 vive bearing the same names ; and it is impossible for the most 

 critical species maker to distinguish one from the other. 

 • Even their varieties, and montrosities, or abnormal forms, 

 are still repeated."* Dr. Gray, however, does not feel satis- 

 fied with the bare examination of the shell in geological 

 formations ; he must have the shell, the operculum, and the 

 teeth; and as ''none of these except the shell can be examined 

 in the fossil state, their position in the various genera must 

 be always attended with more or less uncertainty."! Other 

 competent observers, both on the Continent and in this 

 country, share this opinion. 



Cuvier founded his primary divisions of the mollusca on 

 their locomotive organs, and thus obtained the names Cele- 



* J. Gwyn Jeffreys, 'British Conchology,' 1865. 



Dr. Morch, of Copenhagen, says — " A monographic research, chiefly 

 based on the teeth of the genera Nassa, Fusus, and Bucciuum, found on the 

 coast-lines from tlie Arctic regions to the equator, would probably be sufla- 

 cient to prove whether species in each fauna are created originally, or are 

 only varieties dependent on different climates, and would at the same time 

 prove the relations between species of succeeding geological periods." — 

 Ann. Mag. of Nat. Hist., n. ser., vol. xvi, p. 388. 



t Dr. J. E. Gray, 'Ann. Mag. Nat. Hist.,' ser. 2, vol. x, p. 413. 



