314 HELIX. 



have founded colonies in climates foreign to them, outnumber the 

 colonized members of all other Helicoid genera together. 



The causes of this adaptability are obscure. Perhaps the rather 

 unusual toughness of the external integument and the unrivalled 

 complication of the geuitalia are factors of importance, the first 

 allowing them a wider range of station with greater variety and 

 opportunity of feeding, the second producing more perfect eggs. It 

 is noteworthy that the dentition is of a very generalized type, show- 

 ing no tendency toward the specialization seen in the radulse of 

 Polymita, Oxychona, Papuina, or the entire series of genera group- 

 ing around Acavus, Helicoplianta and Panda. Such high modifica- 

 tion of dentition as these genera show, must restrict them to the 

 special conditions and food which produced it, and would constitute 

 a bar to their wide dispersal, which is not present in the genus 

 Helix. The jaw is of high type, but the same efficient odontogna- 

 thous form has been developed in many genera. 



With the exception of Euparypha and Eremina, no divisions of 

 Helix can be based upon anatomical characters, for the features 

 intergrade throughout, offering merely specific differences. The 

 various " sections " of the genus rest wholly upon conchological char- 

 acters, which though quite appreciable to the eye, are often extremely 

 difficult to define in words so that they may be distinguished. 



The genus Helix is abundantly represented in the Tertiary depos- 

 its of middle Europe, by species belonging without doubt to the 

 modern groups, although in many cases they are practically inter- 

 mediate between some of the latter. The HELICOGENA or Pomatia 

 group is not known with certainty below Pleistocene deposits, 

 although it is barely possible that the Oligocene H. globosa Sowb. 

 belongs here. I do not think this likely ; and the evidence at hand 

 indicates that the group arose upon non-European soil, and spread 

 northward or northwest in a few specific forms which have split in 

 comparatively recent times into numerous species. TACHEA, how- 

 ever, has an extensive range in time, a considerable number of forms 

 appearing in lower Miocene deposits, some showing certain features of 

 Iberus, others with more conoidal spire than usual in normal recent 

 Tacheas, but still having the characteristic columella and band pat- 

 tern. H. bohemica, H. moguntina and H. hortulana are examples* 

 being the " Coryda " of some European authors, so-called on account 

 of the trifling incident of a raised spire. Such forms as H. crepidos- 

 toma Sandb., with keeled earlier whorls, are also to be regarded as 



