100 THE NAUTILUS. 



elevated ; the anterior part of the mussel is more rapidly and directly 

 tapering to a rounded point, the color is light grayish with concen- 

 tric, irregular, narrow zones of a darker shade. Some forms have 

 resemblance with P. splendidulum, the more rhombic ones, with a 

 corresponding form of P. pauperculum, but in case of doubt, the thin 

 shell and very fine hinge with the markedly straight, slender lateral 

 teeth will distinguish our species. 



Specimens have occasionally been seen for several years, from 

 different places, both recent and fossil, yet it seemed not safe to 

 establish a n. sp. upon them. The recent finds of Messrs. Walker 

 and Sargent have put an end to all doubts. 

 Plsidiura monas, n. 



Minute, rather well inflated, oval in outline, without any project- 

 ing angles, except a very slight one at the scutum, and the slightly 

 pointed, rather inferior, anterior end ; beaks little posterior, broad, 

 rounded, little elevated ; surface with comparatively coarse, micro- 

 scopic, rather regular stria?, tops of beaks smooth and shining ; shell 

 thin, hinge fine, cardinal teetli very small, almost straight, longi- 

 tudinal, the posterior (upper) of the left valve sometimes almost 

 obsolete ; ligament small. 



Long. 1.7, alt. 1.4, diam. 0.9 mill. 



Habitat : Mountain Lake, Marquette county, Michigan, collected 

 by Mr. Bryant Walker. 



Among the specimens seen (some twenty), little variation was 

 noticed ; the species may be recognized by its minute size and its 

 shape, the low, comparatively broad beaks, and the somewhat coarse 

 striation. The latter is about as in Pis. punctatum, small forms of 

 which are also of about the same size; but the latter Pisidium is 

 more inflated, more angular in outline, its beaks are narrower and 

 more elevated. P. monas is also smaller than imbecille, and less 

 elongated, its striation is coarser. 



Being known from but one locality, the species has been estab- 

 lished only after a most careful examination and comparison with 

 P. punctatum, imbecille, harfordianum, handiverkii, as well as with 

 the young of the other species occurring in the same lot. 

 Pisidium streatori, n. 



Of medium size, almost equipartite, short oval-rounded to almost 

 circular, without any projecting angles, or with a slight, rounded one 

 t the scutum, moderately inflated, lentiform ; beaks slightly pos- 



