xlii GEOGRAPHIC DISTRIBUTION. 



Gl'tndinidce and Streptaxidce, which, indeed, may have originated in 

 America. On the south and south-east, the Oriental area of Epi- 

 phallogona overlaps somewhat that of the much older Austral fauna 

 of Endodontidce, Rhytididce etc., which lies mainly south and east 

 of the range of the other group. Similarly, the Epiphallogona extend 

 southward far beyond the range of the Belogona. The succession 

 of these faunas from south to north in this Asio-Australian belt of 

 islands, is extremely significant, and clearly indicates the compara- 

 tive ages of the groups in that region. The chronological order of ap- 

 pearance of Endodontidee, Macroogona, Epiphallogona and Belo- 

 gona, as determined by theoretical grounds from their comparative 

 anatomy, coincides with the evidence given by their geographic dis- 

 tribution. 



Belogona. By comparing the organs of such an Epiphallogonous 

 form as Chloritis (pi. 28, figs. 1-4) with some Asiatic or American 

 Belogona, such as Monadenia, pi. 59, figs. 81, 86, or Mastigeulota, 

 pi. 66, fig. 26, it will be noticed at once that the structure of the 

 male genitalia is identical in the two groups; each having a short 

 penis continued in an epiphallus which bears the retractor and ends 

 in a flagellum. The female side is alike in the two groups in hav- 

 ing the spermatheca duct long and branchless, the other organs 

 being identical except that in the Belogonous groups the dart ap- 

 paratus is added. The jaw, teeth and shell show no features diagnos- 

 tic of the groups Epiphallogona and Belogona. It is, therefore, 

 highly probable that the latter group originated from the former, 

 merely adding the dart apparatus to the characters already pos- 

 sessed by the parent stock. There is no especial reason for believ- 

 ing that this transformation took place in any other area than that 

 now occupied by the most nearly allied modern forms of each of 

 these groups, viz. southeastern Asia or the adjacent island groups. 

 The evidence derived from comparative anatomy tends to show that 

 the dart apparatus of the Helices was evolved de novoin this group, 

 and while analogous to that of the Zonitidce, it is not homologous. 

 As in Zonitidce, the glands associated with the dart sack were origin- 

 ally proliferations from that sack ; and this structure is still retained 

 in the Oriental and American genera constituting the BELOGONA 

 EUADENIA. In the European group of genera the glands have 

 moved from the dart sack to the vagina, and are generally found 

 inserted above, never below, the insertion of the dart sack. This is 

 a purely secondary change, and together with the modification of 



