THE JAW IN PULMONATA 



21 I 



circular bites inflicted by a schoolboy upon his bread and butter. 

 The jaw of Helix (Fig. 107, B) is arched in shape, and is 

 strengthened by a number of projecting vertical ribs. That of 

 Limax (A) is straighter, and is slightly striated, without vertical 

 ribs. In Buliinuliis (C) the arch of the jaw is very conspicuous, 

 and the upper edges are always denticulated ; in Orthalicus there 

 is a central trangular plate, with a number of overlapping plates 

 on either side ; in Succinea (E) there is a large square accessory 

 plate above the jaw proper. The form of the jaw is peculiar not 



Fig. 107. — Jaws of 

 various Pulmoii- 

 ata : A, Lima.c 

 [gagates Drap. , 

 Lancashire, x 15); 

 B, Helix [acutis- 

 sima Lam., Ja- 

 maica, X 15) ; C, 

 Jjidimidus (de- 

 pict us Reeve, 

 Venezuela, x "20); 

 D, Achatina 



(/wZicaFer., Mau- 

 ritius, X 7) ; E, 

 Succinea (ehgans 

 Kiss., Aral Dis- 

 trict, X 30) ; F, 

 Limnaea {stag- 

 nalis L., Cam- 

 bridge, X 30). 



only to the genus but to the species as well. Thus the jaw of 

 H. as2)ersa is specifically distinct from that of H. 2)omatia, and 

 that of H. nemo7'alis is distinct from both. Wiegmann has 

 observed^ that in young Arion, Limax, and Helix, the jaw 

 consists of two pieces, which coalesce by fusion in the adult, 

 thus indicating a stage of development in advance of the double 

 jaw which is found in most of the non-pulmonate Mollusca. In 

 all fresh-water rulmonata there are two small accessory side 

 plates besides the jaw proper (Fig. 107, F). 



Nearly all the non-carnivorous rrosobranchiata, land, fresh- 

 water, and marine alike, are provided with two large lateral jaws. 



1 Jahrh. Deal. Malak. Gcsell. iii. p. 193. 



