LYMN^EID^: OF NORTH AMERICA. 307 



The above table shows that there is some variation in the measure- 

 ments of the different organs. The genitalia of palustris differ from 

 those of the smaller Lymnseas (Galba) in the form of the prostate, 

 which is ovate in Galba while in Stagnicola it is squarely rounded at 

 the anterior end and gradually tapers toward the posterior end. The 

 penis is also thicker and larger in Stagnicola. 



The size and number of eggs in the egg-capsules of Galba palustris 

 are as follows : 



Length. 



Width. 



Eggs. 





12.50 mill. 



3.50 



mill. 



82 



Illinois. 



13.00 " 



3.50 



" 



68 



" 



16.00 " 



4.50 



" 



50 



Colorado. 



19.50 " 



4.50 



" 



64 



" 



11.50 " 



4.50 



" 



26 



" 



Range (Figure 33) : Circumboreal. Northern Asia and Europe. 

 North America from the Atlantic to the Pacific Ocean, and from 

 Alaska south to New Mexico. 



Palustris probably inhabits nearly the whole of North America 

 from the Arctic Circle to southern California and New Mexico. It 

 appears to be absent from the Lower Austral (Austro-riparian divi- 

 sion). It may be said to be characteristic of the Boreal and Tran- 

 sition life zones, from whence it has penetrated into the Upper Austral 

 life zone. Its absence from Labrador, Ungava and the northern part 

 of this region may be accounted for by lack of records. Although 

 reported from these regions by Dall (Alaska Moll., p. 76), no speci- 

 mens from these areas have been examined by the writer, nor have 

 any definite rcords been seen. It is possible that its place in this region, 

 •as well as in Greenland, is taken by Galba vahlii. West and north- 

 west of Hudson Bay it is common and widely distributed. Its hypso- 

 metric distribution extends to 9000 feet in Colorado, 7760 feet in 

 southern California, and 10,000 feet in the Uinta Mountains, Utah. 



Comparing the range of palustris with the drainage regions, we 

 find it absent from the Labradorian, Alaskan, Carolinian, Lower Mis- 

 sissippian and Central American regions. It enters the Rio Grandian. 

 The neighborhood of the 40th parallel marks the southern boundary 

 in this region, excepting a small portion of Illinois. In the west, 

 palustris extends almost to the 30th parallel, the altitude providing the 

 temperate climate of the northern portion of the Mississippi Valley. 

 The general northwesterly extension of the distribution indicates ap- 

 parently the origin of the species in America, it being an emigrant 

 from Asia via Bering Strait. Additional records will doubtless estab- 

 lish an unbroken route from the Arctic shores of Alaska to the Great 



