COSMARITM. 71 



c. concinnum, Rab., more elongated, semi-cells nearly 

 quadrilateral, all angles truncate, or rounded. 



Diameter 9-26 yw. 



Found frequently ; probably has a wide range north, south, 

 and west. 



d. simplicissimum, Wille. Plate XLIII, figs. 11-13. 

 A variety of forma octangularis, Wille. 



Pond near Amherst, Massachusetts. 



O. OCTOGONUM, Delp. Var. CONSTRICTUM, Lagh. Plate XLTX, 



figs. 34, 35. 



Cells in front view oblong-tetragonal deeply constricted ; 

 semi-cells trapezoidal ; in vertical view elliptic with middle 

 somewhat inflated ; isthmus one-third of diameter of cell. 

 Membrane smooth. Var. constrictum is more deeply con- 

 stricted, sinus wider and semi cell in vertical view not 

 inflated. 



Tewksbury, Massachusetts. From these specimens the 

 significance of the name is not evident. Instead of three 

 crenulatious or angles on each side of the semi-cell, Delponte 

 describes "sometimes four," or eight to the semi-cell. Has 

 some resemblance to C. Braunii, Reinsch, but is not so regular. 



C. POLYGONUM, Naeg. Plate XIX, fig. 30. 



Small, polygonal, about as long as wide, entire, sinus very 

 narrow; semi-cells hexagonal-oblong, angles subacute or 

 obtuse | apices straight or rounded ; cytioderm smooth or 

 punctate. 



Diameter 15-20 yu. 



Ponds, New Jersey and Massachusetts. Not frequent. 



. LUNATTJM, Wolle. Plate XIX, fig. 16. 



Cells nearly as long as wide, circular, constriction deep 

 and forms a wide rounded or oval sinus ; semi-cells lunately 

 curved, back circularly arched, base concave ; angles of the 

 two semi -cells, more or less acute, approximate closely. 

 Cytioderm punctate ; isthmus usually less than one-third of 

 the diameter. End view elliptical ; side view circular. 

 Diameter 25-28 /<. 



Brown's Mills, N. J. 



. ACULEATUM, Wolle. Plate XIX, fig. 15. 



Medium size, suborbicular, length slightly less than the 

 diameter: constriction deep, forming, by the incurving of 

 the angles of the semi-cells, two elliptical sinuses ; cytioderm 



