THE ANATOMY OF PLEUROTOMARIA BEYRICHII. 235 



of the odontoplioral cartilage ; it pulls the radula forwards 

 and inwards. 



The remaining muscles only act indirectly upon the radula 

 through the movements of the odontophoral cartilages. One 

 of these lies at the side of the main cartilage, to Avhich it is 

 attached by a long fleshy insertion (fig. 30 G, 1. 1, m.) ; it then 

 runs back as a flat muscular band, and takes its origin from 

 the outer border of the basal cartilage. The contracture of 

 this pair of muscles causes the anterior ends of two main 

 cartilages to diverge, and so tends to flatten the anterior 

 part of the radula. The second of these two muscles is 

 situated ventrally, and is the only unpaired muscle in the 

 buccal mass ; it consists of a transverse band of fibres running 

 from the outer border of one main cartilage to the corre- 

 sponding surface of the other, and thus by its contraction 

 approximates the cartilages (fig. 30 G and H, v. t.m.). 



The odontophoral cartilages are four in number. Of these 

 two are very large and laterally compressed, constituting the 

 main cartilages which support the radula. The remaining 

 two are the small basal plates presenting concave surfaces 

 for articulation with the former. In spite of the small size 

 of the basal plates, they appear to be the relatively fixed 

 points for insertion of the majority of the muscles of the 

 buccal mass. 



The radula itself will be considered later. 



Owing to the complicated nature of the movements of the 

 radula we commonly find that the muscles of the odontophore 

 are similarly complicated. Unfortunately it is not easy to 

 ascertain with any degree of precision the exact nature of the 

 movements produced by the contraction of a given muscle, 

 and consequently it is inadvisable in the present state of our 

 knowledge to give them very precise names. 



It is interesting to find that the arrangement of the odonto- 

 phoral muscles of Pleurotomaria compares very closely 

 with that described as occurring in Patella by Geddes.^ 



' "On the Aleclianism of the Odontophore of certain MoUusca," 'Trans. 

 Zool. Soc.,' vol. X, 1879. 



