THK NAUTILUS. 65 



species, this Iteiiig the (livi(liii<r line between the two f.iunal regions. 

 (R.) 



In this locality the Uniones are normal in shape, but very much 

 smaller than in other places in Delaware and the neighboring states. 



Adult specimens of Unio complanatus measure 42 to 52 mm. long 

 (the normal size being 70 to 95 mm. in other Delaware streams). 



JJ . jisherianus is 40 to 43 mm. long, against 95 to 108 mm. at 

 Choptank Mills. 



Lampsilis cariosus, 40 to 44 mm. long. 



Lampsilis ochraceus, 40 to 50 mm. long. 



Lampsilis radiatus, 42 to 48 mm. long. 



Lampsilis nasutus, 50 mm. long. 



The whole Unio fauna is thus dwarfed, the shells being from about 

 half to two-thirds the ordinary size. (P.) 



It has occurred to me that the dwarfing of the Unionidae of the 

 Nanticoke may be due to the fact that no tributary of this sluggish 

 river runs through a soil furnishing ingredients favorable to shell 

 growth. The relative sluggishness of this whole river system may 

 also have the degenerating effect of non-resistance. (R-) 



In the following list a few Delaware shells collected by Mr. Wit- 

 iner Stone are included, such records being duly credited : 



Polygyra tridentata (Say). Brandy wine Valley between Dupont* 

 and Rockland, Delaware. 



Polygyra albolabris (Say). Brandywine Valley between Duponts 

 and Rockland, and hills southwest side Brandywine near Paline, 

 Delaware. (W. Stone!) 



Polygyra thyroides (Say). BrandywMue Valley between Duponts 

 and Rockland ; southwest of Brandywine Hills just below Penn. 

 State line ; and near Delaware City, Delaware. (W. Stone !) 



Polygyra kirsuta (Say). Brandywine Valley between Duponts- 

 and Rockland, and near Delaware City, Delaware. 



Polygyra mo7iodon fraterna (Sny). Brandywine Valley between> 

 Duponts and Rockland, Delaware. 



Strobilops labyrinthicus (Say). Brandywine Valley between 

 Duponts and Rockland ; near Dover, Kent Co.; also near Delaware 

 City. 



Bijidaria contracta (Say). Near Delaware City, Delaware. 



Gircinaria concava (Say). Brandywine Valley between Duponts 

 and Rockland, Delaware. 



