82 THE NAUTILUS. 



of growth were collected in the Fox River, Wisconsin, by the late 

 Geo. T. Marston. 



Yar. rostratum n. Of medium size, moderately to rather well 

 inflated, oblique, outlines slightly angular ; beaks little prominent, 

 rounded or with slight ridges around the slightly flattened central 

 areas; angles at scutum and scutellum well projecting, slightly 

 rounded, especially at the latter, the projecting part of the mussel is 

 somewhat rostrum-like, pinched, surface with fine, irregular striae, 

 somewhat glossy ; color pale to yellowish or brownish-horn, shell 

 rather thin, translucent, hinge slight, with the teeth small but well 

 formed. 



The typical form (of the var.) from Reed Lake (Dr. Kirkland), 

 Blue Lake, and other waters in Michigan, Clear Lake, Indiana 

 (Daniels). 



Var. arrosum n. Rather small and low, not very oblique, outlines 

 somewhat angular, moderately inflated, beaks not much elevated, 

 narrow, rounded or slightly flattened on top, without appendages, 

 and with barely any radial lines ; surface with very fine striae above, 

 becoming somewhat more distant and irregular towards the margins, 

 slightly shining, color pale to yellowish or reddish-horn, shell thin, 

 somewhat translucent, hinge slight. 



Michigan : Grand Rapids in several lots (Streng, Kent Scient. 

 Inst.), Blue Lake, Allegan Co. (Dr. Kirkland), Brooks Lake, 

 Newaygo Co. (Streng), Green Creek, Allegan Co. (Walker). A 

 peculiarity of this rather constant form is a disease found on the 

 shells of almost all specimens : small white spots of erosion gradually 

 becoming confluent and extending over a large part of the surface. 



Var. confertum n. Of medium size, somewhat oblique, well and 

 compactly inflated ; beaks broad, not much prominent, slightly 

 flattened on top, or rounded, without appendages; upper margin 

 moderately curved, with projecting angles where passing into the 

 supero-anterior and posterior slopes which are well marked, rather 

 long, straight, the former steep, the latter nearly perpendicular ; 

 scutum and scutellum marked in some specimens ; surface with 

 irregular, slight striae and lines of growth, dullish to slightly shining; 

 color straw to yellowish, reddish or brownish-horn, generally with 

 irregular, lighter and darker, concentric zones ; shell rather stout, 

 slightly translucent, nacre whitish, muscle insertions well marked, 

 hinge moderately stout, as compared with typical compression, median 



