The Non-Marine Mollusca of Portuguese East Africa. 



185 



Opeas vengoense Conn., 1922. 

 (Plate IV, fig. 26.) 



1922. Opens vengoense Conn., A.M.N.H. x, p. 120. D. 



Hab. L. Marques. Mount Vengo, 5500 ft. (Cressy). 



A minute shell, occurring at considerable depth in decaying vegetation. 

 Its .smaller form and comparatively longer last whorl distinguish it from 

 the young of the next species. 



Opeas cressyi Conn., 1922. 

 (Plate IV, fig. 27.) 



1922. Opeas cressyi Conn., A.M.N.H. x, p. 120. D. 



Hab. L. Marques. District N. of Macequece (Cressy). 



The beautiful little shell selected as type measures long. 10-8, lat. 2-8 ; 

 apert. alt. 3-3, lat. 1-2 ; last whorl 5-7 mm. It is the largest of a bewildering 

 series which may eventually prove to contain more than one species. Egg- 

 holding, and therefore nearly mature, shells range from the type downwards 

 to a length of only 5 mm., while individual examples appear to differ in 

 relative convexity of whorl, distance between sutures, and closeness and 

 depth of striation. There seem, however, to be such exact intermediates 

 between each extreme that I can only conclude that they are all members 

 of one species and that the variation is due partly to local conditions of food 

 and weather and partly, perhaps, to individual reversion to earlier and more 

 distinct types, of whose promiscuous interbreeding the present confused 

 race is the result. 



I append rather a lengthy table of measurements of certain specimens, 

 as it appears advisable to show the convergence between extremes of form. 



last whorl 5-2 mm. 



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