96 Hogg, on the Lingual Membrane of Mollusca. 



basis of classification. There is also a work published in the 

 German language, of all others the most valuable as a book 

 of reference, it is by Dr. Troschel, of Bohn.* Upon the 

 value of such a system of classification I beg to oiFer a few 

 remarks. 



Although the patterns or types of lingual membranes 

 appear to be, on the whole, remarkably constant, " yet," 

 says Woodward, " their systematic value is far from uniform. 

 It must be also remembered that the teeth are essentially 

 epithelial cells, and, like other superficial organs, liable to be 

 modified in accordance with the wants and habits of the 

 creatures. The instruments with which animals obtain their 

 food are of all others most subject to those adaptive modifica- 

 tions, and can never, therefore, form the basis of a true 

 system. t" Di'- Gray, however, on the other hand, has such 

 confidence in the permanence and importance of the teeth in 

 the economy of these animals, that, " if any considerable modi- 

 fications appeared in those of two genera which had been 

 referred to the same family, or much more of two species 

 which had been referred to the same genus, it should be 

 concluded that they had been erroneously placed in such 

 close proximity, as this modification must indicate an im- 

 portant difference in the habits and manners of the living 

 species under consideration Avhich had before escaped obser- 

 vation.":}: Professor Loven, of Stockholm, in a paj^er on the 

 miollusca of Scandinavia, proposed to divide the lingual 

 bands into fourteen groups, and separate the genera into 

 families and sections, characterised by the number, position, 

 and forms of the teeth ; adding, " that the teeth, like the 

 oj^erculum, have usually a structure characteristic of the 

 genera or subgenera, and remarkably uniform throughout 

 some whole families or groups of families." Dr. Troschel, in 

 terms most decided, says — " That if all else were gone, the 

 teeth would afford a reliable means of distinguishing species, 

 and that even the minute differences exhibited in closely 

 allied genera cannot fail in being of great value in the 

 discrimination of critical species." The following table 

 gives the last arrangement proposed by Troschel and Gray : 



1. Tanioglossa. (Tooth formula 3 — 1 — 3.) 



Litorina, Natica, Triton, &c. 



2. Toxoglossa. (F. 1—0—1.) 



Conus, Terebra, &c. 



* 'Das Gebiss der Sclinacken zur Beriinducg einer Naturlichen classifi- 

 cation.' Bohn, 1856—1858. 



t ' Woodward's Manual,' p. 450. 



X ' Ann. Mag. Nat. Hist.,' ser. 2, vol. x, p. 413. 



