Reference List of South African Non-marine Mollusca. 243 



GENUS ISIDOKA, Ehrenberg, 1831. 

 (Symb. Phys., Evertebrata.) 



Type of Genus, I. hemprichii, Ehrn. (contorta, Mich.). 



Kobelt (1909) is probably correct in placing in this Genus all 

 South African species formerly ascribed to Physa. Every one, 

 whose animal has been examined, has proved to belong to Isidora, 

 and it seems unlikely that Physa exists in the subcontinent. 



The Genus has suffered severely for a long time from misidenti- 

 fication. It is the merest truism to remark that the speed of a 

 stream and the nature of its soil have so great an effect on the shell 

 of its rnolluscan inhabitants that colour, epidermal sculpture, and 

 even substance or contour are of little account in the determination 

 of fresh-water species ; yet many have been identified, and others 

 founded, on these features alone, without reference to the form of 

 the parietal and columellar regions, which appear to offer the only 

 moderately stable basis for comparison. I regret that the results of 

 my investigation so far have been of a purely destructive nature, but 

 fear that even more of this will be necessary before it is advisable to 

 describe the one or two new species which possibly exist in South 

 Africa. Mr. E. A. Smith has kindly assisted me in the examination 

 of such species as are now, for the first time, brought into synonymy. 



516. ISIDORA ANGOLENSIS (Morelet). 



1866 Physa angolensis, Morel., J. de C. xiv. p. 162. D. 



1868 Voy. Welwitsch, Moll. p. 88. pi. 9, 



f. 8. D.F. 

 1873 ,, algoensis ,, Sow., Conch. Icon. pi. 7, f. 53. D.F. 



(Err. tijp.) 



1886 ,, angolensis ,, Cless., Conch. Cab. p. 334. pi. 47 



(1885), f. 2. D.F. 



Type in British Museum. 



Hab. DAMARALAND (Geale). 



Described from Angola. 



Specimens of angolensis received by the British Museum from 

 Geale in 1869 were incorrectly labelled algoensis, which gave rise to 

 Sowerby's use of the latter name. Judging from the figure, 

 /. parietalis (Mouss.), from Ovampoland, must be nearly allied to 

 this species. It agrees in form, and the parietal plate, from which 

 Mousson derived the name, is a frequent, and inconstant, feature in 

 other members of the Genus. 



