270 HYGROMIA. 



shell and of the genital organs, and from Eulota which has a shell 

 of much the same form and texture. 



In certain forms (H. revelata, H. tiliata, and the section Meta- 

 Jruticicola) the dart sack and mucus glands are absent ; but as there 

 are other species showing the gradual steps of this loss, first in the 

 empty condition of the dart sacks, then their disappearance, and 

 finally the gradual disappearance of the mucus glands also, we are 

 compelled to consider these simplified species as degenerate and 

 secondarily simple lines of descent It is noteworthy that in shell, 

 jaw and teeth they retain the normal structure of the genus, as well 

 as in the structure of the penis. 



The presence of these forms lacking the cardinal features of the 

 Belogona might be construed by some as invalidating the premises 

 upon which the primary groups of Helices are founded ; but this 

 would be a very short-sighted view. The facts simply show that in 

 some members of highly organized groups, retrogressive evolution 

 has taken place, resulting in structures similar to those character- 

 istic of lower groups. This is a very common phenomenon in many 

 orders of animals. In the case under discussion, the organs of mas- 

 tication and the shell have undergone no changes, and the penis and 

 its appendages retain their normal characters. Compare v. Ihering, 

 Morphol. u. Syst., p. 450, who supports this view. 



In regard to the nomenclature adopted for the group, I have 

 simply made the changes from current usage demanded by the law 

 of priority. It is absurd to continue to use " Fruticicola " in a 

 generic sense when it is everywhere acknowledged that Hygromia is 

 a dozen years earlier, and is properly diagnosed, etc., in a work 

 known to and used by all systematic conchologists. Fruticicola is 

 later than Monacha (type incarnata) and on a par with Bradybcena. 

 The names Zenobia, Petasia, Trochiscus, Latonia and Triehia are 

 clearly preoccupied, and can, therefore, have no place in the no- 

 menclature of Helices. They have hitherto been used in ignorance 

 of this fact, or in defiance of it. 



The sectional scheme proposed below is remodelled from current 

 European usage, except that Theba and allied groups have been re- 

 moved to Helicella, as advocated by von Ihering, and the preoccupied 

 names are dropped. The species herein referred to sections Mon- 

 acha and Fruticicola require much investigation, and doubtless con- 

 siderable re-arrangement, and some students may consider it best to 

 split them into more sections. The other sectional divisions agree 



