264 



THE CHICAGO ACADEMY OF SCIENCES. 



humilis. On the north it barely enters the Boreal (Canadian) life 

 zone. It occupies all of the Canadian, Upper Mississippian and Nova 

 Scotian regions, and a large part of the Columbian, Coloradoan, Cali- 

 fornian and Rio Grandian regions. It barely enters the Hudsonian 

 region on the north. Its absence from the Great Plains area is note- 

 worthy, but may be due to lack of records. 



FIG. 25. 



a LowER CALIFORNIA: Common in the ponds of Sierra Laguna, at some 

 height. Before known as far south as latitude 31 51' (Cooper). 



GEOLOGICAL RANGE 2 (Figure 26) : Pleistocene. 



Comparing the geological distribution of modicella with the recent 

 d ; stribution, we find that it covers about the same territory. To the 

 south the recent distribution extends farther into the Lower Austral 

 life zone and to the north it penetrates the Boreal life zone. It is 

 noteworthy that no fossils have been recorded from deposits in the 

 latter zone, the species in Pleistocene time seeming to keep within the 

 Transition life zone. This restriction of distribution may be, and 



1 This record is doubtful as no specimens of true modicella have been 

 seen from Mexico or Lower California. The specimens were probably Galba 

 bulimoides cassi, which somewhat resembles modicella. As the record has not 

 been substantiated, it is excluded from the map, though recorded here for com- 

 pleteness. The original specimens were lost in the earthquake which de- 

 stroyed the Museum of the California Academy of Sciences. 



2 It is probable that a number of records here listed were really based 

 on Galba parva or some one of the related species, as all of the smaller Lym- 

 nseas have been lumped under humilis. It has been impossible to verify the 

 majority of these records and they are, therefore, listed under modicella. 



