196 Transactions of the Royal Society of South Africa. 



Hah. L. Marques. Itschongove (fide von Martens) ; Lebombo Marsh, 

 Rikatla {riippelli) ; Nyiwan ; gardens, Lorenzo Marques ; Makulane 

 (Junod). 



Long series of large Planorbes from the above localities have all proved 

 identical with Plan, pfeifferi Krs., and it may be accepted that the inclusion 

 of the Abyssinian Plan, ruppelli in Junod's list was due to misidentification ; 

 the latter species can therefore be removed from the local and South African 

 lists. 



Burnup is the first to have observed that at a certain stage in the growth 

 of Plan, pfeifferi the aperture is occasionally denticulate, resembling in this 

 respect that of Plan, alexandrinus Ehrn. This phase seems to be most 

 prevalent in examples about 6 mm. in diameter, but is of extremely rare 

 occurrence, a very large number of specimens examined having so far only 

 yielded half a dozen showing the dentition, which appears to become 

 absorbed in more mature shells. Its arrangement is pretty constant in 

 the only 4 examples I have seen. It consists of a small plait in the centre 

 of the paries, with a minute tubercle below it, and 3 small horizontal plaits, 

 of which the lowest is the most prominent, at nearly equal intervals apart, 

 on the interior of the last whorl some distance within the aperture ; in one 

 case there is a 4th, very small plait above the others. These 4 shells 

 measure between 5| and 6 mm. in diameter and hail from such interdistant 

 localities as Umbogintwini, Natal (Burnup) ; near Victoria Falls, Rhodesia 

 (Soper) and Rikatla (Junod). 



This particular dentition is identical with that sometimes found in 

 Planorbula alexandrina (Ehrn.), but in the latter the dentition is far more 

 usual in the immature stage of growth, and is subject to so much variation 

 that it is impossible to deal with it further in this paper. 



External features of animal and pallial organs. — The animal is grey, 

 except for a varying number of irregular patches of deep black pigment 

 on the roof of the mantle-cavity. The foot is rather broad, and rounded 

 at the hind end. The eyes are at the inner bases of the tentacles, which 

 are long and slender, with a semicircular flattened lobe on each side at their 

 outer bases. The male opening is situated at the posterior corner of the 

 lobe on the left side, while the female opening is beneath the broad 

 mantle-edge. 



Below the orifice of the mantle-cavity, which is situated towards the 

 left side of the animal, there is a broad lobe incompletely divided into two 

 portions (PI. VIII, fig. 17). The right-hand portion is broadly rounded, and 

 of a paler colour than the other ; it forms the lip of the so-called pulmonary 

 siphon. The left-hand portion projects further, and is bluntly pointed ; 

 it constitutes the branchial lobe. It is not folded transversely as in Isidora, 

 but bears a longitudinal fold on its upper surface, which is a continuation 



