52 A MANUAL OF AMEEICAN LAND SHELLS. 



fices ; finally, by the absence or presence and character of the jaw, and 

 the character of the lingual dentition. 



When a genus is numerous in species I have, for the sake of conven- 

 ience, adopted sections or subgenera, founded on special features of the 

 shell, such as the absence or presence of internal laminae or tooth like 

 processes within the aperture. 



In treating the species I have recognized a wide range of varia- 

 tion rather than distinct specific weight in the differences one observes 

 among numerous individuals. It must especially be borne in mind 

 that there is always a great difference in size in individuals of the 

 same species, in the comparative elevation of the spire, globoseness of 

 the body whorl, absence or presence of tooth-like process on the pari- 

 etal wall of the aperture, closing of the umbilicus, &c. And it must 

 freely be acknowledged that individuals are frequently met with 

 which cannot satisfactorily be identified, so nearly are they related to 

 several species. 



Dr. Fischer divides the Geophila thus : 



MONOTREMATA. 



Common or contiguous external male and female orifice. 

 Agnatha. — No jaw. 



Gnatliojpliora. — Holognatha : Jaw without accessory piece. Elasmo- 

 gnatha : Jaw with accessory piece. 



DiTREMATA. 



External male and female orifice widely separated. 

 Terrestria. — Terrestrial in habit. 

 Aquatica. — Marine. 



I have modified the descriptions of Fischer where it has seemed nec- 

 essary to me to do so. 



A. — MONOTREMATA. 

 AGNATHA. 



Family TESTACELLID^. 



Animal limaciforra or heliciform; no jaw; lingual membrane greatly 

 developed, surrounding a powerful muscle, formed of oblique rows of 

 elongated, narrow, aculeate teeth. 

 Olandina. 



