190 Transactions of the Royal Society of South Africa. 



Family PLANORBIDAE. 



Subfamily ISIDORINAE. 



Genus Isidora Ehrenberg, 1831. 



I retain this name provisionally in preference to Bulinus of Miiller 

 because the type of Isidora is a known species, whereas no authentic examples 

 of Bulinus senegalensis Muller are in scientific circulation. As soon as 

 recognisable topotypes of the latter come to hand, Miiller's genus-name 

 will undoubtedly supersede some other, but until they are rediscovered it 

 appears inadvisable to introduce it too hastily into current nomenclature. 



Isidora natalensis (Krs.), 1841. 



1841, '43. Physa natalensis Krs., Kiist., Conch. Cab. (Limn.), p. 8, pi. 1, 

 figs. 12-14. D.F. 



Hab. L. Marques. Lebombo Marsh, Rikatla ; Monguane ; Makulane 

 (Junod) ; Beira (Cawston) ; L. Zandemela (Lawrence). 



The examples from the last two localities are very dwarfed. 



Isidora forskali Ehrn., 1831. 



1831. Isidora for skalii Ehrn., Symb. Phys., Evert. 3rd sp, D. 

 1848. Physa wahlhergi Krs., Sudafr. Moll., p. 84, pi. 5, fig. 13. D.F. 

 Hah. Mozambique. Quilimane (Stuhlmann). 

 L. Marques. Beira (Cawston). 



Subgenus physopsis Krauss, 1848. 



Krauss difierentiated his Physopsis africana generically from other 

 South African species of Isidora simply because the latter are perforate, 

 without columellar truncation, while P. africana was imperforate, with 

 truncate columella. Although the columella of Physopsis, however, is 

 always more or less truncate, the absence of perforation is of no generic 

 value, since even in africana clearly perforate examples may be sometimes, 

 though very seldom, found, while other species, such as globosa Morelet, 

 are almost invariably perforate. As pointed out hereinafter by Watson, 

 the anatomy of Physopsis does not afford sufficient ground for generic 

 distinction from Isidora, and it appears more rational to treat it as a sub- 

 genus of the last named, separable therefrom by reason of its more or less 

 truncate columella. 



