10 THE NAUTILUS. 



3. Pis. milium Held, conforming with European specimens, have 

 been collected in Straits Lake, Michigan, by Mr. Bryant Walker, 

 and in Dallas Lake, Stearns County, Minn., by Mr. H. E. Sargent. 

 Among a number of fossils from a marl bed in Tuscala County, 

 Mich., also collected by Mr. Walker, there was one valve of the 

 same species. 



4. In Aroostook County, Maine, Olof O. Nylander has found a 

 few specimens of a Pisidium identical with one from England named 

 milium Held, which, however, Mr. Clessin says, is not that species. 

 It is of somewhat the same shape and color, but larger and the beaks 

 are more prominent. 



The question whether these Pisidia be native or introduced is an- 

 swered, at least for milnim, by the fossil found. As to No. 4, an im- 

 portation is rather improbable, from its habitat. Eor Nos. 1 and 2 

 the possibility of a colonization cannot be denied, just as Bitltynia 

 tentaculata has immigrated from Europe, and, on the other hand, 

 Calyculina transversa Say seems to have been transported into Eng- 

 land. Further researches are very desirable, and also fossils should 

 be secured wherever such may be obtainable. 



5. Pi's, contorl'iim Pr. This Pisidium has been described, in 1852, 

 as a fossil sp. from Massachusetts, and has lately been collected 

 from marl beds in Maine by Mr. Nylander, and in Michigan, by Mr. 

 Walker. Last year Mr. Nylander succeeded in finding a few recent, 

 living specimens, in Aroostook Co., Me. The shell is transparent, 

 of a deep wine or amber color, the surface highly polished. 



ti. Pis. mediamim, n. sp. Mussel of rather small size, elliptical in 

 outline, much inflated, often of somewhat irregular growth; superior 

 and inferior margins moderately curved, posterior well rounded, or 

 with a slight angle above, anterior rounded or slightly truncated ob- 

 liquely; beaks rather in the middle, slightly directed toward the 

 posterior, rather high, prominent over the hinge margin ; scutum 

 and scutellum very slightly marked; surface with very fine, crowded 

 stria\ somewhat shining, light horn to yellowish or straw colored ; 

 shell thin, nacre colorless, muscle insertions barely perceptible; 

 hinge fine, plate narrow ; cardinal teeth lamellar, slightly curved, the 

 right one in its posterior part somewhat thicker, simple or with a 

 fine, longitudinal groove; lateral teeth pointed, the outer ones of the 

 right valve comparatively large ; ligament fine, long. 2.5 to 3.5, alt. 

 .2.0 to 2.8, diam. 1.7 to 2.3 mill. 



