THE NAUTILUS. 125 



may be gained of the way the various species ruQ by the num- 

 bers of specimens taken, recorded in parentheses after each 

 species. In 1914 I kept a fairly accurate census of the Uving 

 specimens collected (first number in parenthesis), and in 1915 

 did the same for the drift specimens, and these figures indicate 

 fairly well the relative abundance of the different forms. All 

 this drift material (listed in the second number in parenthesis) 

 was taken from three quarts, gross measure, or one and a half 

 pounds of fine flotsam, scooped up from one little pocket on the 

 bank of the Musselshell River. 



Mollusca of Winnecook, Montana. 



Vallonia costata montana Sterki (88 ; 390). 

 Pyramidula cronkhitei (Newc. ) (68; 98). 

 Vitrea hammonis (Strom.) (5 ; 22). 

 Zonitoides arborea (Say) (70; 15). 

 Zonitoides minuscula (Binn.) (0 ; 41). 

 Euconulus fulvus (Mull.) (12 ; 4). 

 Agriolimax campestris (Binn.) (7 ; 0). 

 Pupilla blandi (Morse) (0 ; 4). 

 Vertigo binneyana Sterki (7 ; 81). 

 Vertigo ovata (Say) (0 ; 1). 

 Cochlicopa lubrica (Miill.) (8 ; 14). 

 Succinea avara Say (1 ; 9). 

 Succinea oregonensis Lea (1 ; 0). 

 Succinea retusa Lea (21 ; 0). 

 Lymnaea caperata Say (unrecorded ; 12). 

 Lymnaea palustris Miill. (unrecorded ; 1). 

 Lymnaea parva Lea (0 ; 19). 

 Planorbis antrosus Conrad (taken in 1914). 

 Planorbis parvus Say (0 ; 11). 

 Planorbis umbilicatellus Ckll. (0 ; 10). 

 Physa gyrina Say (taken 1914). 

 Aplexa hypnorum (Linn.) (taken 1914). 



The list brings out a somewhat curious mingling of eastern 

 and western forms at a point which is well to the eastward of 

 the Continental Divide. The intimate association of apparently 



