162 Annals of the South African Museum. 



the doubling of the ectocone ; bases of centi'al teeth usually longer 

 than their cusps. Reproductive system : free oviduct not much 

 swollen ; receptacular duct more than twice the length of the free 

 oviduct, crossing the front end of the common duct ; vas deferens 

 keeping to the left of the free oviduct, usually loosely bound to the 

 penis, not terminating in an epiphallus ; penis longitudinally folded 

 inside, and lined by diagonal rows of very minute papillae. 



Distribution. — Usually in the vicinity of rivers in the sandy 

 deserts of Great and Little Namaland, extending eastward into 

 Bschuanaland and northward through Damaraland. 



Genotype Dorcasia alexandri, Gray. 



Dorcasia is undoubtedly very closely allied to Trigonephrus. The 

 two genera, however, can be distinguished externally, both by the 

 sculpture and lip of the shell, and by the footsole of the animal ; 

 while internally they differ in their digestive, reproductive, and 

 excretory systems, the difference in the radula being the most 

 striking (see PI. IV., figs. 9-16). 



On the whole Dorcasia seems to be a rather more highly specialized 

 genus than Trigonephrus, and it is probably not quite so old. It has 

 therefore been thought advisable to describe Trigonephrus first and 

 Dorcasia afterwards. 



DoKCASiA coAGULUM (von Martens). 

 (PI. Ill, f. 1. PI. IV, f. 5, 14, 22, 26, 30. PL V, f. 6.) 



1889 Helix coagulum, von Mts., Sitz.-Ber. Ges. Nat. Fr. Berlin 



p. 160. D. 

 1897 ,, von Mts., Archiv f. Naturg. Ixiii. i. p. 37. 



PL VII, f. 11-14. D.F. 



Shell rather large, subglobose, narrowly umbilicate, thin, trans- 

 lucent, pale corneous above and at sides, with irregular blotches and 

 streaks of opaque cream ; apex brown ; base and peristome white ; 

 interior colourless, exhibiting the markings of the exterior. Spire 

 somewhat produced, apex roundly obtuse. Whorls 5^, rounded, 

 rather gradually increasing, all but the 1^ apical covered with close, 

 faint, regular, transverse striae, becoming much fainter beneath. 

 Suture simple, of moderate depth. Aperture truncate-ovate ; 

 peristome slightly reflexed, ends joined by an extremely faint 

 callus. Umbilicus very deep and narrow, somewhat strangulate. 



Dimensions of a specimen from Fielding's Chabeesies, nearly 

 agreeing in shape with those of the Type set: Diam. maj. 22-8, 



