PLANORBIS. 911 
26. PLANORBIS PARVUS, Say. 
Hald. Monog. p. 27, t. 4, f. 19—23. 
Planorbis Billingsii, Lea. Plate 7, fig. 10. 
Mr. Lea’s species is said by its author to 
differ from parvus in having a wider umbili- 
cus, more oblique aperture, and exhibiting one 
more whorl; but in neither of these respects 
does it differ from the majority of fully grown 
specimens of parvus. 
27. PLANORBIS ARCTICUS, Beck. 
Plate 7, fig. 11, 12, 18. 
Shell corneous or yellowish, widely umbili- 
cate; whorls three and a half, canaliculately 
sutured, the last one obsoletely angulate at 
the periphery ; aperture oblique. 
Diam. 5, alt. 1:5 mill. 
Greenland. ; 
I have not seen this species, which is said 
by Morch to resemble both P/. parvus and 
Pl. deflectus. 
o (58) 15 
