PILSBRT : PHTLOGENT OF AMONIDJ^. 103 



DisTEiBUTioisr OP AmoNiD^, Past and Present. 



The geographical range of the family Arionidse is cliscontinuous, 

 with three widely separated areas : the European, with its highest 

 development in the West of Europe ; the Himalayan ; and the West 

 American. These three centres have no genera in common, nor does 

 any genus occur in more than one of them. 



The West American centre supports the greatest number and 

 variety of genera, including all known Binneyinge and Ariolimacinse 

 (five or six genera) and two of the six genera of Arioninse. The 

 Himalayan area has only the genus Anadenus, which, as we have 

 seen, is' the least differentiated genus of Arioninse. The European 

 centre has the three genera Avion, Letourneuxia, and Geomalacus, all 

 decidedly more nearly related to each other than to any other genus, 

 and forming the most modified group of the family. 



It appears from these data that all the primitive genera are 

 American, including one, Binneya, which in its spiral, external shell 

 with sculptured nepionic whorl, its short body- cavity and solid tail, 

 may reasonably be regarded as a eormecting link with the Endodontidse. 

 All non-American genera belong to the most divergent subfamily of the 

 group. An American genesis for the Arionidse is therefore an extremely 

 probable theory ; and we may with good reason hold that all the more 

 important modifications of the stock, including the differentiation of 

 the three subfamilies, took place upon American soil. The emigration 

 to Asia via an Alaskan land-bridge may well have been contem- 

 poraneous with the immigration of Belogonous Helicidse into America 

 by the same route. The Arionine incursion into the Palsearctic 

 region was not spent until its western limit was reached ; but the 

 Asiatic immigrants have subsequently been exterminated, with the 

 single exception of Atiade?iiis, partly perhaps- by the increasing rigour 

 and dryness of Asiatic climate north of the Himalayas.^ From the 

 sole Asiatic survivor, which probably represents the most southern 

 outpost of the incursion, it seems likely that at the time of the 

 westward spread of Arioninse from America, the group still was 

 characterized by the well-developed penis of the early Arionidae ; 

 and that the degeneration of the penis with the concomitant shifting 

 of its function to the atrium, vagina, or oviduct, has been a later 

 modification, undergone independently by American and European 

 genera. Such a conclusion is- based, not alone upon the morphology 

 of the geographically intermediate genus Anademis, but upon the 

 fact that an appreciable diversity is apparent among the genera ; in 

 Ario7i the free oviduct having assumed the function of the lost penis, 

 while in Geomalacus it is the modified atrium, in Prophysaon the 

 vagina.^ In the Arioninfe which reached Europe, as in the Belogonous 



^ It remains to be seen whether other Arionine slugs may not still exist in some 

 part of this vast and little-known area. 



^ There are other instances in abundance of the loss of penis and assumption of its 

 function of copulation by other organs : Limax primitivus, many species of Cepolis^- 

 some Papuinas, etc., etc, 



