BELOGONA-SIPHONADENIA. 235- 



BELOGONA SIPHONADENIA. 



Dart-bearing Helices in which the mucus glands are tubes of 

 equal diameter throughout, inserted directly upon the vagina, never 

 upon the dart sack. 



This definition, while it perfectly distinguishes the group under 

 consideration from the Euadenia (p. 175), in which the mucus ap- 

 paratus consists of glandular lobes, flat or globular, and with few 

 exceptions inserted on the dart sack, will not cover all forms refer- 

 red to Siphonadenia. The diagnosis-defying process of retrogres- 

 sive evolution or degeneration has produced forms in which the dart 

 apparatus and mucus glands have dwindled to a mere vestige, or 

 been entirely lost ; reverting to the condition found in the Epiphal- 

 logona, which as I have elsewhere attempted to show, were the stock 

 whence Belogona arose. In these cases recourse must be had to 

 such other organs as have not shared the degenerative process; to 

 less divergent species, and to embryology for clues to the true his- 

 tory of doubtful forms. We cannot too strongly insist upon the rec- 

 ognition of that great difference between a primitive structure and 

 similar structure produced by a reverse process from a more com- 

 plicated organ. To lose sight of this would be to lose the best mes- 

 sage these studies can bring us, and reduce systematic zoology to a 

 mere index. 



Fortunately, we have in the recent fauna, a considerable number 

 of species showing clearly the various stages of degeneration which 

 have resulted in those simplified forms of the genera Helicella and 

 Hygromia which will be found noticed in the account of those groups. 

 The evidence indicates that such forms as Ciliella, Metafruticicola, 

 Cochlicella, etc. are recent degenerate groups, quite independently 

 produced from at least four normal Belogonous types. It is note- 

 worthy that the penis, jaw, radula and shell show no retrogressive 

 features in these forms, but retain the characters normal for the 

 genera they are believed to have descended from. The penis is not 

 (as vonlhering states) of the Patula type (Haplogono), but is dis- 

 tinctly of the form normal in Belogona and Epipliallogona. 



All recent Helices of Europe (except the Pyramidulas) belong to 

 this division of the Belogona, and the same is probably true of the 

 Tertiary fossil forms. Just as anatomical data have enabled us to 

 eliminate the foreign group Triodopsis from this fauna, so more 

 philosophical study must cause us to see in the supposed Corasia, 

 Chloritis, Obba, Pella, Charopa, Mesodon, Coryda, etc. of the tertiary, 



