162 



GUY C. ROBSON 



cuticle just below the mandibles on each side. There is also 

 usually a median dorsal projection, dagger-shaped in transverse 

 section. This also stains sharply with eosin, but less intensely 

 than the mandibles. 



Behind this projection are to be found at about the same 

 level in the mouth on either side two glandular patches of 

 unknown function which run backwards to the origin of the 

 salivary ducts. 



Posteriorly to the mandibles the mouth expands laterally 

 and is flattened dorso-ventrally over the lingual cartilages. 



Text-fig, 2. 



P. ventrosa. Transverse section through buccal bulb, c, oral 

 cartilage ; cm, circular muscles ; lin, longitudinal muscles ; 

 hm, basal membrane of radula ; r, radula ; sg. salivary gland. 



In this area it shows in transverse section three main divisions — 

 a median, unpaired cavity with a thin roof, dorso-lateral 

 expansions with ciliated and glandular walls into which the 

 salivary ducts open, and ventro-lateral expansions which dip 

 down beside the cartilages. These- have a cuticular lining. 

 The ciha of the dorso-lateral cavities no doubt serve to circulate 

 the saliva. In P. ulvae Henkiug considers their function 

 consists in driving the food particles backwards. In V i t r e 1 1 a 

 and Bythinella the cilia are continued on to the roof of 

 the median portion. 



The lingual cartilages (Text-fig. 2) correspond to 

 those found in other genera of the family, and in general to the 



