14 GASTROCOPTA, NORTH AMERICA, WEST INDIES. 



warm temperate parts of North America across the tropics 

 to Argentina, while Albinula has not penetrated into South 

 America. The two or three Mexican forms of Albinula and 

 Vertigopsis are northern species which have apparently 

 migrated southward rather recently, as they have not been 

 differentiated from the northern stocks. 



Known Albinulas are from two widely separated regions, 

 eastern North America and central and western Europe. 

 These may be looked upon as the extreme eastern and west- 

 ern herds of an Albinula stock once (Lower Oligocene or 

 earlier) spread over western America and northern Asia, 

 regions where it is now extinct. As we would expect in so 

 ancient a group, the few recent species are structurally iso- 

 lated, representing the termini of as many collateral phyletic 

 series. The groups are as follows : 



Series of G. armifera (species no. 1). Recent, eastern 

 North America. 



Series of G. quadriplicata (species nos. 46 to 51). Middle 

 Oligocene to Pliocene, Europe. 



Series of G. contract a (species no. 2). Recent, eastern 

 North America. 



Series of G. holzingeri (species no. 3). Recent, eastern 

 North America. 



Key to American Species. 



a. Angulo-parietal lamella with lobed, sinuous edge, not 

 much bent and not forked anteriorly; shell large, 3.3 to 

 4.5 mm. long. G. armifera, no. 1. 



a 1 . Angulo-parietal very long, bent L-shaped; shell ovate- 

 conic, 2.3 to 2.5 mm. long. G. contracta, no. 2. 



a 2 . Angulo-parietal lamella forked in front; shell cylindric, 

 about 1.75 mm. long. G. holzingeri, no. 3. 



Series of G. armifera. 



The armifera group is distinguished by its large, oblong 

 shell and the columellar armature. There is a thick, obliquely 

 descending or protractive lamella on the axis, which recedes 

 again towards the base (pi. 1, fig. 3). This is variously modi- 

 fied in the several races. Sometimes the forwardly descend 



