38 GASTROCOPTA, NORTH AMERICA, WEST INDIES. 



well up in the mountains, where thousands of specimens can 

 be obtained by sifting. The single record (Univ. of Colo. 

 Studies, iv, p. 171) of G. pilsbryana from Colorado (Trini- 

 dad), was erroneous; the specimens are typical 0. pentodon. 

 The record of this species from Chicorico canyon near 

 Eaton, N. M. (Nautilus, xiv, p. 83) was also an error for G. 

 pentodon. 



Subgenus IMMERSIDENS Pilsbry and Vanatta. 



Immersidens PILS. & VAN., Proc. A. N. S. Phila., Dec. 11, 

 1900, p. 606 (for B. ashmuni and B. perversa). PILSBRY & 

 FERRISS, Proc. A. N. S. Phila., 1910, p. 136. 



Gastrocoptas in which the angular lamella is united with 

 the parietal at the inner end of the former, the two diverging 

 forward, and together shaped like the Greek letter A, or like 

 a reversed y. The basal fold, when present, is radial, or 

 transverse to the cavity. There are two palatal plies, not 

 standing on a callous ridge, the lower one often deeply im- 

 mersed. Peristome thin, expanded. Shell cylindric, thin, pale. 



Type G. ashmuni (Sterki). 



Distribution, central plateau and mountains from the 

 Grand Canyon of the Colorado, northern Arizona, southward 

 to southern Brazil and western Argentina. 



The area of Immersidens in the United States has been 

 rather well explored, so that few more species are to be ex- 

 pected. The occurrence of the group in central Mexico and 

 Guatemala gives ground for the belief that forms will be 

 found over much of the Mexican plateau. In South America 

 true species of Immersidens occur as far south as Rio Grande 

 do Sul (G. iheringi) and Argentina (G. dicrodonta), and it 

 is likely that G. pazi (Hid.) also belongs to this section. 



Most species of Immersidens daub the shell with dirt, like 

 the species of Albinul-a, and unlike the cleaner typical Gas- 

 trocoptas. 



Key to Species of Immersidens. 



a. Inner end of the parietal lamella strongly curving towards 

 the periphery ; teeth large. 



&. Siuistral ; last whorl straightened and produced 

 forward. G. perversa, no. 8. 



