INTRODUCTION. xxi 



genera and species, as to the arrangement, number, 

 and form of the teeth with which it is armed. This 

 curious and very interesting organ consists of a mus- 

 cular tongue furnished with plates upon which the 

 teeth are arranged in transverse and longitudinal 

 rows. The plates are divided into three areas ; the 

 central one is termed the rachis, and the two lateral 

 or side areas are called the pleurce ; the teeth on the 

 former are called median (central), or rachidian, those 

 on the latter uncini. 



The number and arrangement of the teeth may be 

 illustrated by numeral figures ; the following formula, 

 which represents the system in the lingual ribbon of 

 Helix aspersa, will serve as an example — 52 . i . 52. 

 The figures denote that each transverse row contains 

 one central or rachidian tooth, with fifty-two 2incini or 

 lateral teeth on each side of it ; and as there are, in 

 this instance, one hundred and thirty-five rows, the 

 complete formula stands thus : — 



— = 14,17 K teeth. 



135 



The lingual ribbon of molluscs is well worthy of 

 investigation, for, apart from its affording characters 

 which are useful in determining the position which 

 some genera and species should occupy, it forms a 

 most beautiful and interesting object for observation 

 under the microscope. Being unable to use that in- 

 strument for any length of time without inconvenience 

 or even risk to my eyesight, I have unfortunately been 

 prevented from examining, except in a very cursory 



