During the months of September, October, November and December 1917 I collected 1250 different specimens of fresh-water snails from various parts of the Transvaal, more especially from the sources of the Vaal and Crocodile rivers. All the larger specimens I submitted to microscopic examination for possible infection with cercariae. From the mud at the bottom of the Mooi river at Potchefstroom, which is a tributary of the Vaal, I came across some very finely preserved specimens of Unio caffer Krauss, a large number of which contained small pearls attached to the shell. The amount of lime in the bed of this river probably accounted for the perfect state of the umbones of these Uniones. Amongst the small stones at the bottom of the river I collected some remarkably large specimens of Corbicula radiata Parreyss. The river also contained some specimens of Pisidium, a much smaller variety, not yet identified. On the decomposing rushes at the edge of this river there were numerous specimens of Ancylus, for which Mr Henry C. Burnup has suggested the name Ancylus cawstoni; I have seen similar specimens at Klerksdorp and in the Hex river at Kustenburg, a branch of the Little Crocodile. All these specimens were free from cercarial infection.