The Non-Marine Mollusca of Portuguese East Africa. 



115 



Gonaxis {Eustreptaxis) vengoensis Conu., 1922. 

 (Plate IV, fig. 2.) 

 1922. Gonaxis {Eustreptaxis) vengoensis Conn., A.M.N.H. x, p. 114. D. 

 Hah. L. Marques. District 15 miles N. of Macequece (Cressy). 

 A fine species, showing little variation in a good series collected at 

 different times in various parts of the district. Immature shells are helicoid 

 in form, roundly umbilicate, and bluntly keeled at the periphery, below 

 which there is hardly any sculpture. 



The animal differs from that of Gonaxis {Eustreptaxis) elongatus (Fulton) 



Text-fig. -2.— Gonaxis {Eustreptaxis) vengoensis Conn., Macequece. 

 Representative teeth from the radula ; X 100. 



in having numerous dark spots scattered over the roof of the mantle-cavity, 

 whereas in S. elongatus there is only a single dark patch near the respiratory 

 orifice In most other respects, however, the animals of the two species 

 seem to be very similar, and they are probably fairly closely related. Their 

 radulae are remarkably like each other, although in the present species the 

 cusps of the inner teeth seem to be slightly more curved and somewhat 

 longer relatively to the length of the bases of the teeth. The number of 

 teeth in each transverse row is 26+0+26. They gradually increase m 

 size from the first to the sixth tooth on each side, and dimmish again from 

 about the thirteenth tooth to the outer edge of the radula. In a full- 

 grown specimen the sixth to the thirteenth teeth may all attain a length of 



about -4 mm. (text-fig. 2). , ^ a ■ 



The character of the radula shows that this species is better placed m 

 the subgenus Eustreptaxis, rather than in the closely related genus Marcoma ; 



