xlvi GEOGRAPHIC DISTRIBUTION. 



Tkersites, Obba, Planispira, Papnina, Ganesella, being character- 

 istic of all portions of the tract. The Belogona have a smaller range 

 southward, but in the genera Helicostyla, Eulota and their allies, ex- 

 tending over the central and northern portions of the region. The 

 several genera of Macroogona, such as Helicophanta and Ampelita 

 in Madagascar, Acavus in Ceylon, Panda, Pedinogyra, Anoglypta, 

 etc. in Australia and Tasmania, have a much broken, discontinuous 

 range, indicating a high antiquity and much extinction ; but the 

 origin of the group from Protogonous ancestors, within the general 

 region now covered by the several genera, is probable. 



In conclusion : We find that the distribution of Helices in space 

 and time is not hap-hazard or erratic, as has been supposed from the 

 earlier classifications, and from the erroneous generic and subgenenc 

 references contained in works on the fossil forms, but that it is 

 orderly and comprehensible. We find that, whenever the data are 

 sufficient for judgment, the genera and species of any given life- 

 area exhibit such affinities to each other and to those of adjacent 

 areas, that no orographic changes or continental extensions other 

 than those recognized by geologists as either demonstrated or prob- 

 able, are necessary to account for the various snail faunas of to-day. 

 We find that not only is it unnecessary to throw land bridges across 

 the depths of Atlantic and Pacific to account for the distribution of 

 Helices, but that such hypotheses are contrary to many facts indicat- 

 ing that such groups of snails as are common to America and Europe, 

 have radiated from an Oriental center westward to Europe and east- 

 ward via the Bering Sea route to America, while in the far south 

 a hypothetical extension of the Antarctic continent fulfils the con- 

 ditions asked by the zoogeographer. Another fact worthy of remem- 

 brance is that in each faunal region, one or a few types of Helices 

 have been modified to fill the several stations available, and that the 

 most highly modified forms are generally found to be nearest akin 

 to the normal Helices of the same region, not to similarly modified 

 Helices of other regions. Thus, the groups Pliengus, Papuina, 

 Oxychona and Leptoloma are strikingly similar, yet they are not 

 related to each other, but to less abnormal snails occupying their 

 several areas. The same is true of Caracolus and Thersites ; 

 Camcena, Euhadra and Hadra, Stylodonta and Columplica, Isognomo 

 stoma and Triodoptit, and scores of other groups. 





