40 AMANUAL OF AMERICAN LAND SHELLS. 



is colored pink, tlie Central Province blue 5 tbe Eastern Province (of 

 wliich tlie northern i)ortions are not shown) is uncolored. The subdi- 

 visions, or regions, of the Eastern Province are also indicated by col- 

 ored lines. The red line marks the division between the Northern and 

 Interior Eegions. From this line the last-named region extends (its 

 subregion of the Cumberland shown by green lines) to the brown and 

 yellow lines, wbich, taken together, mark the northern boundary of the 

 Southern Region, the yellow separately indicating the Texan Subregion, 

 the brown the Floridan Subregion. 



In the above pages I have simi)ly stated the facts now known regard- 

 ing the actual distribution of our land shells, scarcely attempting to ex- 

 plain it. I will here venture to make a few suggestions on this subject. 



The student of geographical distribution must now take as his guide 

 the recently published work by Wallace on this subject.* From this 

 he will learn that terrestrial mollusca of most of the recent genera have 

 existed on the globe from very early geological times. Also, that, 

 wherever originally appearing, their universal distribution over all the 

 continents is easily explained. Thus we readily account for their pres- 

 ence in Forth America,t and, however imj^erfect may be the geological 

 record, it shows us that at least Zo7iites, Pupa, Helix, Bulimulus, Vi- 

 trina, Macroeyelis, and Clausilia existed here in previous geological 

 ages. From these ancestors, no doubt, have been derived, through 

 many intermediate stages of development, the present fauna. I have 

 already shown that the characteristic American genera of the Eastern 

 Province, the Mesodon, Triodopsis, Stcnotrema, &c., were already estab- 

 lished in Post-pleiocene days. It is impossible to learn how much ear- 

 lier they appeared, but of one significant fact we are certain — they are 

 more recent than the elevation of the Rocky Mountains and Sierra Ne- 

 vada, for otherwise these chains would not form, as now, dividing lines 

 between the Eastern, Central, and Pacific fauna. There are, indeed, 

 several small species which have passed these barriers, being found 

 over all of North America. These same species are found equally dis- 

 tributed in Asia and Europe. They are undoubtedly of much earlier 

 origin, than the strictly American species, and belong to some extinct 

 fauna of world-wide distribution. The circumpolar connection of the 



* The Geographical Distributiou of Animals, with a Study of the Eelations of Living 

 and Extinct Fauuas.as elucidating the past Changes of the Earth's Surface. By Alfred 

 Russell Wallace. Amer. ed. Harper & Brothers. New York. 1876. 



t In the following pages it will be seen that three well-established genera only — Hetn- 

 pliiUia, I'rophysaon, and ArioUmax — are jjcculiar to our limits, excepting perhaps a 

 few disintegrated Helix. 



