Baker — Notes on a Collection of Mollusks. 6 



Superorder Eatbynenra. 



Order PULMONATA. 



Suborder Basommatophora. 



Family Physidae. 



The collection of Physa and Lyranaea made by Dr. Nason 

 is exceptionally fine. The first-named genus is represented 

 by a large number of specimens and embraces some eight 

 species and varieties. This genus is in a bad state of chaos, 

 hardly any two authors agreeing as to what limits shall be 

 assigned to the different species. The various species are 

 but little understood by the majority of conchologists, most 

 of the species being lumped under heterostropha and gyrina. 

 As a matter of fact the Physac may be, for the most part, 

 easily distinguished when carefully and intelligently studied. 

 They are much less variable than the Lymnaeas, the specific 

 limits being easier to determine. Such characteristics as the 

 form of the aperture and outer lip, the spire (acute or dome- 

 shaped), sculpture, and the presence of various bands mark- 

 ing rest periods, are valuable points in the separation of the 

 species. 



A careful study of the genus Physa, founded on abundant 

 material from widely separate points, would do much to un- 

 tangle this interesting group of shells. Mr. O. A. Crandall 

 made a splendid beginning in the Nautilus (volume XV), but 

 he was not permitted to finish the work so well begun, his 

 death having occurred just as the last part of his paper was 

 published. It remains for some careful student to take up 

 the work where he left it and prepare an illustrated mono- 

 graph of the " tadpole " snails. 



The following notes are based on an abundance of material 

 and may aid to some extent in straightening out the northern 

 species. 



Aplexa hypnorum Linne. 

 Stony Point. 



