96 DIGESTIVK ORGANS. 



covered by a multitude of siliceous teeth symmetrically arranged 

 in transverse rows. The teeth of the anterior portion of the 

 radula are the most develoj^ed but at the same time more worn 

 than those of its posterior portion which have not yet come into 

 use, and the projection of which above the surface is scarcely 

 discernable. These teeth are thus developed behind and are 

 successively taken into use. In the Littorinidse the lingual 

 plate reaches its maximum development, its length, when uncoiled, 

 being in Tectarius pagodus seven times that of the body of the 

 animal. In Patella, Chiton, Cyclostoma, etc., the odontophore 

 is also relatively ver^^ long. 



For convenience of description the odontophore may be 

 divided into five longitudinal areas, which are crossed by the 

 numerous transverse rows of teeth ; these teeth are distinguish- 

 able in character in each area (x, 11 ; xii, 1.3). The central tooth 

 is termed median or rachidian, the adjoining area on each side 

 bears the laterals or pleura;, and these again are flanked by the 

 tnarginaU or uncini. Sometimes, however, only three areas are 

 found, when the laterals are suppressed, and a cross series 

 includes only rachidian and uncinial teeth. In Bullidaj again, 

 the rachidian teeth are suppressed and there are simply two 

 bands of uncini, whilst in a few gastropods the tongue is 

 unarmed. A numerical formula has been devised which repre- 

 sents these teeth thus : 



In Trochus (oo. 5. 1. 5. co.), meaning 1 rachidian, 5 laterals 

 and numerous ( oo being the sign of infinity) uncini. Muricidse 

 have but three longitudinal areas and the formula for Murex, for 

 example, is 1. 1. 1., signifying one rachidian tooth, with a single 

 marginal on either side. 



The following list, compiled hj Dr. J. Gr. Fischer, will give 

 some idea of the number of teeth developed upon the odonto- 

 phores of several species : 



Eolis Drummondi, 16 (Mobius) ; Eolis papilloma ^ 30 (Mobius) ; 

 Baccinum undatum, 240 (Mobius) ; JVassa reticulata^ 267 (Mo'bius); 

 Fusus antiquits^ 450 (Mobius); Subulina octona, 3025 (Fischer 

 and Crosse); Liitorina littorea, 3500 (Mobius); Doris tubercu- 

 lata, 6000 (Hancock) ;' Limnaea jugtdaris, 8343 (W. Gr. Binney) ; 

 (9«.c^d^eZZa ce/iica, 8384. (Fischer and Crosse) ; Bulimus Gantagal- 

 lanics, 12,100 (id.); Helix aspersa, 14,000 (Thomson); Zonites 

 euryomphalus, 11,473 (F. & C.) ; Helix pomatia, 21,000 (Thom- 

 son) ; Orthalicus longus, 23,660 (F. & C.) ; Limax maximuSj 

 28,800 (Thomson); Tritonia Hombergi, 36,000 (Hancock); Helix 

 Ghiesbreghti, 39,596 (F. & C). 



Whilst the number of teeth (as well as their form) in each 

 transverse row is constant in the individuals of a species, the 

 number of the rows is, of course, variable, according to age and 

 the usage which the ribbon has received ; and the above figures 



