Notes on South African Mollusca. 121 



value in confirming, or correcting, deductions drawn from concho- 

 logical features alone. Even now, however, more than one of the 

 species, of which animals have been obtained, is represented by- 

 only a single spirit specimen, and others are not in the best con- 

 dition, so that the results of their examination are not quite as 

 satisfactory as would have been the case had a more complete 

 representation been available. 



It will be seen that Watson states that Dorcasia and Trigonephrus 

 are nearly aUied to each other, but differ considerably from other 

 genera whose anatomy is known. They are rightly placed by Pilsbry 

 in the Family Acavidac, and a new Sub-family, Dorcasiinae, may 

 conveniently be founded to contain them. 



The Dorcasiinae are distributed in all shapes and sizes along the 

 south-western seaboard of Africa, from Algoa Bay to Damaraland, 

 and in some instances also to a considerable distance inland. 



I must confess that when I first undertook this paper, it was with 

 the idea that many existing species could be shown to be hardly 

 varietally distinct, and that, owing to the presence of connecting 

 links, their names would be scarcely worth preserving. Extended 

 study, however, tends to prove that most of the named forms 

 exhibit quite sufficient and constant difference to entitle them to 

 specific rank ; while, in other cases, shells from the same locality 

 constitute a local race of some species, consistently well defined, 

 and so widely divergent from the Type that a name is advisable 

 in order to prevent confusion. 



From an anatomical standpoint, Watson places the genus Trigo- 

 nephrm first and Dorcasia after it, and as his arrangement of the 

 species, based on their anatomy, is perhaps the most convenient for 

 comparison of the shells, I follow it in the ensuing pages. 



The history of the Acavidae and all subsequent references to 

 anatomical details are in the exact words of Mr. Watson, who has 

 also furnished the figures of the animals, and my warmest thanks 

 are due to him for his kind co-operation. 



I may here mention that I have made no attempt to make 

 my measurements, taken from the actual shells, agree with the 

 figures, all of which are the natural size. Hardly any two persons 

 will be found to measure a globular helicoid in exactly the same 

 fashion, and a very slight accidental difference in the angle from 

 which the shell is viewed or measured will account for a propor- 

 tionate difference in the dimensions recorded. 



As full references to every species were given in my " Revised 

 Reference List of South African Non-marine Mollusca," published 



