136 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



of succinctly stating their numbers and positions, the 

 following method of formulation is used. 



The central teeth of the rachidian series are 

 denominated by the sign 1, when present, and when 

 absent; the admedian teeth by the signs 1,2,3..., 

 according to the number present ; while the lateral 

 teeth are noted by the sign repeated as often as 

 there are lateral teeth on either side ; when the 

 number of admedian or of lateral teeth is very large, 

 the sign x is used in place of 1, 2, 3. .. , or repeated. 

 For example, when, as in ^Eolis, there are no lateral 

 teeth, we write the formula . 1.0; that of Amphis- 

 phyra, is 1 . 1 . 1 ; that of Aplysia, 13 . 1 . 13 ; and that 

 of Oncidium, 54. 1 . 54 (Woodward). 



The whole mass of the odontophore may be of 

 considerable size, and, in the limpet, the radula is two 

 or three times the length of the body ; the number 

 of separate teeth may be very great, as among 

 the snails, where 167 transverse rows of 135 teeth 

 each will give some twenty thousand teeth ; in some 

 species of Helicidae, the aggregate exceeds thirty- 

 nine thousand (39,596). 



The teeth are sometimes large and hooked ; some- 



O ' 



times conical and upstanding ; when the rachidian 

 teeth are, as sometimes happens, absent, another part 

 of the digestive tract may, as in the Bullidse, be pro- 

 vided with calcareous plates which replace them 

 functionally. In a few (e.g. Bhodope) the odonto- 

 phore is lost. 



In a number of cases, the muscles that move the 

 radula are not confined to those that are inserted into 

 the supporting cartilages, but there are others that 

 pass to the walls of the head ; the contraction of 

 these is the cause of the licking movement which a 

 protruded radula may be often seen to perform. 



In some, especially slugs and snails, a hard horny 

 plate is developed on the roof of the mouth cavity, 



