THE NATURAL HISTORY SURVEY. 129 
the left valve is armed with a dorsal long, narrow, lamellar, al- 
most straight tooth, extending about half way to the ventral 
border of the hinge plate, and a large, rounded, pyramidal tooth 
placed near the ventral border; lateral teeth strong, elevated, 
curved, the right valve teeth strongly projecting above the 
valve edge, the left valve teeth not so much so; double in the 
right and single in the left valve; cavity of the beaks deep. 
Length, 3 50; height, 3.00; breadth, 2.50 mill. (9880). 
= 4.50; oh 4.003 fs 200“ (12468). 
Animal: Not observed. 
Distribution: New England west to California, Canada 
south to Arizona and New Mexico. 
Geological distribution: Pleistocene. 
Flabitat: Similar to additum. 
Remarks: The winged beaks are a peculiarity of this species 
which will aid in its determination, although the appendages 
are not always present. Its trigonal form is different from any 
other species of the genus found in the region under discussion. 
It is found in both the western and southern regions. 
50. Pisidium politum Sterki, unfigured 
Pisidium politum STERK1, The Nautilus, Vol. IX., p. 75, 1895. 
‘Mussel of medium size, well inflated, rather high, beaks 
slightly posterior, rather high and prominent, not full but well 
rounded; scutum and scutellum slightly marked. Superior mar- 
gin rather short, rather strongly curved; inferior well curved, 
more so in front than behind; posterior margin distinctly trun- 
cated, with a well marked angle where joining the superior, and 
a less marked, rounded angle where joining the inferior margin; 
anterior end forming a slight but distinct angle situated rather 
high up. Surface very finely, irregularly striated, polished; 
whitish or straw colored, often leaden grayish on the beaks, or 
even all over. Shell moderately thick; nacre whitish; muscular 
insertions not very distinct; hinge of essentially the same type 
as that in P7s. abditum. 
‘‘Length, 4.0; altitude, 3.4; diameter, 2.4 mill.; 4.7; 4.0; 2.9 mill. from 
another place. 
“Pis. politum is rather variable in size, and more so in color 
as noted above, but so markedly constant in its several habitats 
that a number of different local forms, or varieties could be 
described. It has some resemblance with Ps. additum Hald. 
