1908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 817 



and XeritincB. The Helicinidfe occur chiefly in islands in the 

 Indo-Pacific region, and in the Antilles, reaching their maximvim 

 in the last-named locality. As has been already remarked, they 

 are only found fossil in later Tertiary deposits, with the exception 

 of the remarkable DawsoHiella., which occurs in the Carboniferous 

 of Illinois, and is apparently related to Trachydomia, a subgenus 

 of Naticopsis, from the same formation. The present distribution 

 and the geological history of these Pulmonate Neritacea suggest 

 problems which will be dealt with in the second part of this paper. 

 With the exception of Dawsoniella, they are unquestionably 

 descended from Neritid or Neritinid ancestors, and it is difficult 

 to account for their dispersal in islands so widely separated from 

 one another without making assumptions which differ widely from 

 accepted doctrines of animal evolution. 



It Avas my original intention to undertake a monographic 

 account of the anatomy of the Neritacea, but as the work proceeded 

 it became evident that, owing to the difficulty of obtaining suit- 

 able material, and the great number of species which must 

 necessarily be subjected to microscopical examii:iation before com- 

 pleteness conld be attained, this would be a task of many years' 

 duration. The results already attained seem of sufficient import- 

 ance to merit publication, and may induce zoologists travelling or 

 living in tropical regions to give attention to a group deserving 

 more attention than it has received. I therefore purposel}' omit 

 a detailed account of certain organs, such as the buccal bulb, the 

 radula, and the histology of the alimentary tract, the mantle, &c. 

 My chief attention has been directed to the nervous system, the 

 kidney, the pericardial cavity and its connection wdth the rest of 

 the cceloni, and the generative organs. 



The difference between the aquatic Neritidfe and the terrestrial 

 pulmonate Helicinidse is so considerable that they must be dealt 

 with in separate sections of this memoir, and in each case, in 

 order to avoid a confusion of my own observations with a criticism 

 of the work of other authors, I will pi-eface my statements with a 

 short account of the literature of the subject. 



Family Neritid^, 



The first important contributions to the anatomy of Nerita are^ 

 those of Quoy and Gaimard (35 & 36), whose figures and descrip- 

 tions of the nervous system and alimentary tract leave much to be 

 desired, and it is not necessary to enter into an examination of 

 their errors and omissions. But they made some observations on 

 the genei-ative organs which, though far from complete, have not 

 been followed up, and have scarcely been noticed by any subse- 

 quent author except Bergh. It will be best to quote their 

 description in full : — " Dans le sexe femelle est un groupe 

 d'organes qui merite quelque attention. On y voit I'extremite 

 du rectum, puis un corps pyriforme tres-allonge entoure en ]3artie 

 d'une sorte de glande striee en travers, qui s'ouvre vers le bas. 



52* 



