104 GASTKOCOPTA, EASTERN ASIA. 



of the early Oligocene migration westward, when the group 

 nourished in Europe from Middle Oligoceue to Pliocene. 

 Smalbinula is more widely distributed than any other sub- 

 ordinate group of the genus. 



The Asiatic mainland and European Pliocene species are 

 the most progressive, some having the angulo-parietal lamella 

 thoroughly unified, as in typical Gastrocopta. In America 

 no species are referable to Sinalbmula proper, but the section 

 Vertigopsis is evidently a derivative group, differentiated 

 chiefly by the degeneration of both the angular and parietal 

 lamellae, the former being either \vholly lost, or in some forms 

 still present but very minute. 



The Bonin Island group remains primitive in the separation 

 of angular and parietal lamellae, leading us to believe that 

 they have been isolated a very long time. 



The Australian Sinalbinulas are more primitive than the 

 Asiatic continental forms. The angular and parietal lamellae 

 are imperfectly connected, and the columellar lamella is hori- 

 zontal and short, or the inner end is but slightly curved down. 

 The palatal callous is thin but noticeable in species I have seen. 

 They are in about the same stage of evolution as the isolated 

 Bonin Island recent species, and those of the European Oligo- 

 cene, though rather different by the strongly converging in- 

 stead of subparallel angular and parietal lamellae. 



Key to Eastern Asiatic Species. 



a. Three subequal and parallel lamellae on the parietal wall ; 

 Bonin Is. 



&. Length about 2.4 mm. G. chichijimana, no. 38. 



& 1 . Length about 1.75 mm. G. c. ogasawarana, no. 38a. 

 a 1 . Angular and parietal lamellae closely juxtaposed and 

 united by a callous, but not continuous. 



&. Parietal lamella nearly behind the angular ; an infra- 

 parietal lamella shorter than the parietal ; Bonin Is. 



G. boninensis, no. 39. 



6 1 . Angular and parietal lamellae juxtaposed laterally; 

 no infraparietal lamella or tubercle ; Indo-China. 



G. ejecta, no. 45. 



