The Non-Marine Mollusca of Portuguese East Africa. 113 



The buccal ganglia are situated some distance further back, at the anterior 

 end of the odontophore, and are also close together, the buccal commissure 

 being unusually short. 



The muscular odontophore is about 6 mm. long, and is curved as shown 

 in PI. V, fig. 3. The following are its principal intrinsic muscles : Exter- 

 nally th'err is a moderately thick sheath composed almost entirely of 

 muscle-fibres running in a circular direction. When this is cut open, 

 afaother cylindrical structure is disclosed within the outer sheath, the 

 bottom and sides of which are formed of the semitranslucent odontophoral 

 support or " cartilage," while the top consists of a thick compact layer of 

 transverse muscle-fibres uniting the two edges of the support (text-fig.' 1 B). 

 There also arises from the outer edge of the support on each side a longi- 

 tudinal series of slender muscles, which curve downwards and unite with 

 the outer sheath. Towards the front end of the odontophore other slender 

 muscles arise from the same place, but curve upwards and unite with the 

 top of the sheath. These suspensor muscles are dotted in the diagram. 

 The anterior part of the radula is folded over the front of the odontophoral 

 support, and extends backwards beneath its anterior half, in a posterior 

 ventral pocket of the buccal cavity. A broad longitudinal muscle runs 

 back from the hind end of this pocket, as shown in the section and m 

 PI. V, fig. 2. The latter figure also shows the powerful longitudinal 

 retractors of the radula-sac, as seen after the removal of the odontophoral 

 support and the transverse muscles connected with it. Most of these 

 retractors are inserted close to the front end of the narrow radula-sac, 

 which they practically surround. Of these the lowest pair of muscles is 

 the largest, while those above the radula-sac consist of a number of small, 

 separate strands. These latter muscles arise all along the edges of the 

 odontophoral support, but the larger muscles below them seem all to have 

 their origin at the posterior end of the support. In addition to these 

 muscles inserted near the front end of the radula-sac, an unusually stout 

 terminal retractor is inserted at its extreme hind end, and this muscle 

 appears to be continuous with the extrinsic buccal retractor. 



The radula itself is long and rather narrow, measuring about 84 X 1-4 mm. 

 when flattened out. The teeth are all unicuspid, being of the aculeate type 

 characteristic of the family, as will be seen from text-fig. 1 A. The rows 

 form a conspicuous angle in the middle, and no central teeth are present. 

 The teeth towards the middle of each row have somewhat shorter cusps 

 than the others, but on the whole the shape of the teeth is fairly uniform. 

 They gradually increase in size from the first until about the sixth on each 

 side!^ and then become smaller again from about the tenth tooth to the 

 edge of the radula. 



Perhaps the most striking feature of the radula, doubtless mainly due 



