56 



GEOGEAPHICAL DISTRIBUTION. 



before 



genus 



began 



ts first migration; it 



further strengthened by the decrease 



of this percentage along the 

 The si i 



gle 



spt 



various meridians directly with the distance from the pole, 

 of the Siberio-Caucasus meridian, as before, strengthens rather 



ystem that now practically constitutes it, 



than weakens the general evidence. Along the north-west flanks of the mountain 



but that originally limited it on the east 



and south, there was an unbroken continuity of the physical conditions of its original 



ily, but during retreat as well, while communication with 

 till a very late period shut off on one side by this 



home, not during, ad van 



neighbouring meridians 



mountain system and on the other by a sea. 



was 



Uninfluenced from without and 



ith 



a 



continuous 



larity of 



onment, the species of this merid 



retained throughout a 



uniformity of character which disappeared from those 



f the other merid 



These two sections 



to be 



dered separately. Morphological 



der 



ations have led us to conclude that the relationship of the Bidentam to the Anodontje 



is 



of direct descent. But the same consid 



have shown that this is also true 



of the Rhy 



d from the absence of any evidence that the Rhyncholopeue 



have been derived from the Bidentat^, we had to assume that the relationship between 

 these sections is one of collateral descent from the Anodont^e. This presumption, based 

 on the absence of evidence that the Bidentat^e were intermediate between the other two 

 sections mentioned, is now established by the direct evidence from distribution. We 

 now find that the Bidentat^b probably originated at a later date than the Rhyncholoph^e 

 and under somewhat different conditions. 



DIAGRAM 10. 



Distribution of the Bidextatjh. 



DIAGRAM 11. 



Total 

 Percentage 



• • 



54 sp. 



Oil 



/ 







I 



i 





