126 PROCEEDINGS OF THE MALACOLOGICAL SOCIETT. 



Quaternary : Bonneville Lake beds, Utah ; Lahontan and Carson 

 Prison Luke beds, Jlevada ; Owens and Le Conte Lake beds, California. 

 Pliocene : Kettleman, Santa Clara, and Cache Lake beds, California. 

 Miocene : Contra Costa Lake beds, California. 



A. cygnea. Shell similar to preceding, usually somewhat larger, 

 however, elongate-ovate, sub-alate, somewhat broader posteriorly, 

 tapering to a blunt point, moderately inflated, growth to mid- 

 adolescence as in imjmra ; habitat similar. 



Entire Paloearctic Kegion. Alaska, Eraser, Columbia, Klamath, 

 Utah, and Nevada Systems, but occasionally farther to north or south. 



Quaternary : Bonneville Lake beds, Utah. Pliocene : Kettleman 

 Lake beds, California ; Idaho Lake beds, Idaho and Nevada. 



A. cygnea Beringiana. Shell similar to preceding but atteimate- 

 iilliptical, not broader posteriorly, barely alate, strongly inflated ; 

 impura stage pushed back to early adolescence, cygnea stage to mid- 

 adolescence ; habitat same. 



North-Eastern Siberia. Yukon, Alaska, Eraser, rarely in Columbia 

 System. 



The accompanying Plate V illustrates the evolution of this species 

 from vnpura through cygnea to Beringiana. It seems probable that 

 impura is of West Amei'ican origin, and after giving rise to a northern 

 sub-species, cygnea, the latter spread to Asia by a land-bridge during 

 the upper Miocene at a time when the climate was somewhat warmer 

 than at present, and cygnea doubtless occupied all Alaska. Once in 

 Asia tlie extension of cygtiea to Europe and the Mediterranean region 

 has been only a matter of time and facility in taking advantage of 

 stream-captures, etc. There appears to be no evidence that more than 

 the one race is represented in the Old World outside of Kamchatka, 

 though occasional atavistic individuals suggest tmjJura, while others 

 tend to acquire the characters of Beringia7ia. It seems improbable 

 that the European Najadologists would overlook these well-marked 

 sub-species if they existed, so thoroughly has this species been studied. 

 Anodonta cygnea Beri)igiatia appears to be a sub-species of comparatively 

 late origin, arising in Alaska, doubtless, and taking advantage of the 

 early Quaternary land-bridge to migrate to Siberia. Had it originated 

 in Kamchatka or crossed the Bering Straits during the Miocene 

 connexion it would be difficult to explain why it has not extended 

 its range farther to the westward. 



The problem of temperature appears to be an important factor in 

 limiting the north and south distribution of the various suh-species. 

 It is noteworthy that cygnea does not extend in the Old World beyond 

 the latitudes of its extreme limits in the Californian Province. 



The verv young of this species have been frequently obtained in 

 organic mud with Corneocyclas j^ii'^cJtella. 



Genus Goxidea, Conrad. 



Anodonta (sp.), Lea, 1838 {A. angulata, Lea); Gonidea, Conrad, 1857 

 {A. Randalli, Trask = A. angnJata, Lea, first species). 

 Type, Anodonta angulata, Lea. 



