THE NAUTILUS. V 



30°. The lOotb meridian roughly divides the eastern from the 

 western Lymnaeid fauna, only 17 species and varieties being common 

 to both. The highest development of the group is in the glaciated 

 portion of the continent. 



The classification adopted presents several interesting features. 

 Following Dybowski and some other European authors, Mr. Baker 

 separates the Planorhidce and Ancylidse as families distinct from the 

 Lymnmidce, on the basis of differences iu the shell and soft anatomy, 

 which seem amply to justify that view. 



The genus Lymnsea, as hitherto used in this country, is divided 

 into the following genera and subgenera : 

 Genus Lymncea Lamarck, type L. stagnalls. 



" Pseudosuccinea Baker, type L. columella. 



" Radix Montfort, type L. auricularia (Europe). 



" Bulimnea Haldeman, type L. megasoma. 



" Acella Haldeman, type L. haldemani (Z. gracilis Jay). 



" Galba Shrank, type L. truncatula. 



Subgenus Si7npsonia Baker, type L. humilis. 



" Stagnicola Swainson, type L. palustris. 

 " Leptolimncea Swainson, type L. glabra (European). 

 " Polyrhytis Meek, type /-. kingii (fossil). 

 Genus Pleurolimnsea Meek, type L. tenuicosta (fossil). 



The genus Galba contains nearly all of our species, Lymnsea 

 and Pseudosuccinea each having two, BulimncBa and Acella each one 

 species. The genera are based upon quite readily recognized shell 

 features, and rather small but apparently constant differences in the 

 soft anatomy, chiefly the genitalia and teeth. The anatomical differ- 

 ences between the genera are not so great as are generally recognized 

 as generic in the land snails, since they consist mainly of differences 

 in the proportions of certain organs, rather than in the possession or 

 absence of structures ; yet their value in classification depends upon 

 their constancy in long series of species. Many more forms should 

 be dissected with this in view. At present the evidence seems to 

 favor recognition of the several genera in the Lymnaia group defined 

 by Mr. Baker. 



Numerous figures are given of the dentition, jaws and genitalia of 

 American species, — one of the most valuable features of the work. 

 In the terminology of the genitalia we would suggest a change, in 

 order that it may conform to the usage in writings on land pulmo- 



