The Non-Marine Mollusca of Portuguese East Africa. 193 



species. A prominent fold runs along the roof of the mantle-cavity about 

 half-way between the kidney and the rectum, and opposite this there is a 

 similar fold on the rectum itself, the front end of which can be seen below 

 the branchial lobe in PI. VIII, fig. 8. These folds are also found in related 

 forms. But in the present species, and also in /. {Physo'psis) africana (Krs.), 

 another thickened ridge occurs, which runs along the kidney near its upper 

 edge and curves upwards posteriorly on to the roof of the mantle-cavity, 

 wh^re, however, it becomes so low as to be scarcely perceptible. Opposite 

 to this ridge on the kidney there is a slight thickened ridge along the lower 

 edge of the rectum. These two ridges correspond to the much larger ridges 

 which occur in the same positions in Planorbarius corneus (Lin.), but they 

 do not seem to occur in the more typical species of Isidora such as I.forskali 

 Ehrn., /. contorla (Mich.), /. tropica (Krs.), and /. lamellata (Smith). Indeed, 

 the presence of these longitudinal ridges in the mantle-cavity, in addition 

 to the pair of folds above them, seems to be the one marked anatomical 

 feature which distinguishes the subgenus Physopsis from Isidora s.s. 



Central nervous system (PI. VIII, fig. 9). — The nerve-ring is situated behind 

 the buccal mass, and is very small, the cerebral, pedal, pleural, and visceral 

 ganglia being all closely aggregated owing to the exceptional shortness of 

 the connectives that unite them. The arrangement of these ganglia agrees 

 well with that in /. lamellata * but the left parietal ganglion has a well- 

 marked lateral lobe, external to the origin of the left pallial nerve, which 

 Pelseneer does not show in his figure of the nervous system of that species. 

 The buccal ganglia are moderately large, and are united by a commissure 

 about equal in length to the greatest diameter of the ganglia. Laterally, 

 at their junction with the cerebro-buccal connectives, they each give off 

 two large nerves to the sides of the buccal mass ; in front there arise two or 

 three slender nerves to the oesophagus, salivary glands, etc. ; while the 

 odontophoral nerves arise at the inner sides of the ganglia, near the ends of 

 the buccal commissure. 



Digestive system. — The dorsal jaw is rather thick and dark brown, 

 with an obtuse median projection and fine vertical striae. It measures 

 about -9 mm. in length, and nearly -4 mm. broad in the centre, though much 

 narrower towards the ends. The lateral jaws are narrow and weak, and of 

 a much lighter colour. 



The radula (text-fig. 24, A) is of the type found not only in Isidora, but 

 also in Miratesta, Protancylus, Planorbarius, Carinifex, etc. The central 

 teeth are bicuspid, and rather narrow. The lateral teeth, of which there 

 are eight or nine on each side, are broader, and tricuspid, with triangular 

 basal plates ; in some of them, however, the ectocone already shows a 

 tendency to divide. There are about thirty-six multicuspid marginal 

 * Pelseneer: op. cit., p. 369, pi. xvi, fig. 19. 



