180 Annals of the South African Museum. 



thickened, reflexed, but not overhanging the umbilicus, which is very 

 wide and deep, extending to and clearly exposing the transparent 

 apex. 



Diain. maj. 27'9, niin. 22'5 ; alt. 14'1; apert. 16'2 x 13'0 rum. 



Hob. DAMARALAND. Mt. TJsakos (Frames, 1915). Erongo Moun- 

 tains (Rogers, 1914). Bullspoort, between Nauchas and Maltahohe 

 (Tucker, 1916). 



At Mr. Burnup's request I have placed the Type in the British 

 Museum. 



The chief points in which the new variety differs from typical 

 alexandri are in the umbilicus, which is wide and deep instead of 

 shallow and strangulate, and the aperture, which is more acuminate ; 

 the sculpture also, though fine and close, is markedly more pronounced 

 than in the typical form. 



Both its umbilicus and aperture closely resemble those of rar. 

 perspectiva, but the sculpture is so distinct that unless intermediate 

 forms are found there will never be any difficulty in differentiating one 

 from the other. 



I have selected as Type a shell possessing the double advantage 

 of being the freshest specimen, and also almost exactly intermediate 

 between the two extremes in size, for the latter feature varies greatly, 

 the largest example measuring: Diain. maj. 34'5 ; min. 28 - ; alt. 

 16'4 ; apert. 21*1 x 15'6 mm.; and the smallest: Diam. maj. 21'7; 

 min. 17 - 3; alt. lO'O : apert. ITS x 9'7 mm. The average size of the 

 variety, however, would appear to be a little greater than that of the 

 Type, the smaller specimens being in a minority. 



The fact of its only occurring, so far, on mountains permits the 

 choice of a distinctive varietal name, but I do not suppose that 

 montana will necessarily prove to be confined to mountainous districts, 

 or that such surroundings exercise any influence on the characters of 

 the shell. The Type-set were collected at different altitudes between 

 300 and 1200 ft., but the size does not appear to be affected by the 

 height. The largest example comes from the 700 ft. level, and the 

 smallest from that of nearly 1200 ft., but there is no average 

 uniformity, as the highest and lowest levels also produce shells only 

 infinitesiinally smaller than the maximum. 



The aperture is fairly constant in form throughout the series, 

 measuring in four other specimens 19 - 5 x 16, 16 - 8 x 14, 16'75 x 14'3, 

 and 14-8 x 12'3 mm. 



A series of bleached shells in the South African Museum from the 

 Erongo Mountains present very nearly the same characters as the 



