THE NAUTILUS. 99 



margin which is slightly curved, with slightly projecting, rounded 

 angles at the scutum and scutellum, the latter often covered in lateral 

 aspect, in full-grown mussels ; supero-anterior slope well marked, 

 slightly curved to nearly straight, posterior part subtruncate to 

 rounded ; young specimens are generally more angular in outlines 

 than adults ; surface with very fine, crowded, concentric striae and 

 generally a few well marked lines of growth, somewhat shining ; 

 color light to deeper horn, generally with lighter zones along the 

 margins ; shell rather thin, more or less translucent ; hinge slight, 

 well formed, plate narrow, right cardinal tooth moderately to strongly 

 curved, its posterior part thick and grooved ; a deep, impressed 

 groove in the plate between its inferior edge and the cardinal tooth ; 

 left anterior strongly curved, posterior oblique, slightly curved to 

 nearly straight ; " lateral teeth " with short, slight, thin cusps, the 

 left ones and the right anterior pointed, the outer anterior of the 

 right valve distinct, the posterior quite small ; ligament rather slight. 



Long. 3.2, alt. 2.8, diam. 2.2 m. (100 : 37.5 : 69) average. 



Long. 3.o, alt. 3, diam. 2.4 m. 



Soft parts not examined. 



Habitat : Mountain Lake and Pine River, Marquette county, 

 Michigan, on the south shore of Lake Superior, Mono Lake, Musk- 

 egon county, Michigan, collected by Mr. Bryant "Walker. A 

 Pisidium from the Isle Royale, in Lake Superior, and some speci- 

 mens from other places in Michigan, seem to range under the same. 

 Several hundred specimens at all stages of growth, collected in 

 Mountain Lake from 1895 to 1905, are remarkably uniform and 

 characteristic in appearance, although showing slight differences in 

 outline. There is none of the described .-peril- under which the 

 present Pisidium might be ranged, and it is -iniilar only to trape- 

 zoideum, being of about the same size and approximately the same 

 shape. But the latter mussel is much more annular in outline, its 

 supero-anterior slope is steeper, the anterior end is more angular 

 and more below the longitudinal median line, the beaks are narrower, 

 the surface more dull, and tie hinge much stouter. Although 

 known for over tin years, this Pfst'dium has not been published 

 before, as I thought it might be connected with some other species. 



Pis. succineum n. pp. Mussel of moderate size, moderately in- 

 flated, somewhat elongate, little oblique ; beaks slightly posterior, 

 rounded or slightly flattened, somewhat projecting over the upper 



