266 ABIDA. 



bending of the outer lip where the upper palatal plica or fold 

 emerges. 



Many French authors call the lamellae of the parietal wall 

 ' ' superior folds ' ' ( plis superieurs) . 



Distribution. 



While species of Abida occur from Spain to the Balkans, 

 and north to southern England, they are chiefly concentrated 

 about the Alps and the Pyrenees, which have been evolution 

 centers of species and races. A. secale (with its immediate 

 relatives or races) covers nearly the whole range of the genus 

 and extends far north of all other species, but the other 

 groups of species are more local. 



I. PYRENEAN CENTER: Groups of A. polyodon, A. braunii 

 and A. affinis. Three rather closely related groups, allied to 

 the Alpic frumentiana. In this connection it may be noted 

 that SandahUa cylindrica appears to be merely a special modi- 

 fication of the same ancestral stock. A. -polyodon has spread 

 as far east as the Var valley in France, but most of the species 

 are strictly mountain forms of the Pyrenees. The western 

 limit of the genus in Spain has not been worked out. 



II. ALPIC CENTER: Groups of A. frumentum, A. micheli, 

 A. variabilis and A. atracta. These groups are more dis- 

 tinctly differentiated than the Pyrenean. A. frumentum has 

 the most extensive range, second only to that of A. secale, 

 which is perhaps also to be classed as belonging to the Alpic 

 center. Some other forms, such as A. variabilis, overlap the 

 ranges of a few Pyrenean species in southern France, but 

 none extends as far west as the Pyrenees. The related genus 

 Odontocyclas also belongs to the Alpic center. 



The conclusions to be drawn from the distribution of both 

 Abida and Chondrina are in agreement. The Pyrenean (or 

 Hispanic) center is rather a secondary or peripheral center 

 of evolution, so far as the modern forms are concerned. 

 Though remarkably rich in species and races, the differentia- 

 tion is of somewhat more youthful type than around the 

 Alpic center. 



The alleged occurrence of Abida braunii and A. polyodon 



