88 DES.MIDS OF THE UNITED STATES. 



C. RETUSUM, Perty. Plate XXI, figs. 25, 26. 



Small, of nearly equal length and breadth, entire ; sinus 

 deep, linear; semi-cells with subreniform base, sides sinuate 

 and converging to a truncate end ; angles rounded ; end 

 view centrally protruding. 

 Diameter 22-30 p. 



Pennsylvania and New Jersey* 



C. MARGARITUM, Wolle. Plate XXII, figs. 25, 26, 27. 



Suborbicular, from one to one and one-third times as long 

 as wide; sinus narrow linear; semi-cells somewhat semi- 

 circular, margins undulate crenate, or emargiuate ; two or 

 three surface swellings near the base which cause prismatic 

 reflections from the smooth pearl-like membrane. 

 Diameter 22-25 yw. 



Found the first of this form in Splitrock pond, New Jersey,, 

 but occasionally since in other localities of the same State, 

 and in Pennsylvania. 



C. TITHOPHORUM, Nord. Plate XXII, figs. 28, 29, 30. 



Cells about as long as wide, circular, deeply constricted ; 

 sinus an acute angle considerably ampliated (angle about 

 20) ; semi-cells from the base cuneate-semicircular ; end 

 view elliptical with a prominent central elevation ; laterally 

 seen, obovate-circular with a mamilliform prominence on 

 each side. 



Diameter 28 ^. ; length 30 /<. 



Collected this form from ponds on Mount Everett, Mass. It 

 corresponds so nearly with a plant found on the island of 

 Java, described by Dr. Nordstedt, I adopt the name as hi& 

 choice. 



C. HOMALODERMUM, Nord. Plate XX, figs. 19, 20. 



Slightly longer than broad, deeply constricted ; sinus 

 narrow linear with the mouth considerably ampliated ; semi- 

 cells trapezoid with a subreniform base ; sides retuse ; ends 

 truncate, and often retuse ; superior angles obtuse, inferior 

 broadly rounded ; seen from the vertex, elliptic-oblong with 

 the middle somewhat inflated; lateral view ovate. Ends 

 about half as wide as the breadth of the cell. Cytioderm 

 finely, often indistinctly, punctate. 

 Diameter 40 f.i. 



Ponds, Mount Everett, Mass. 



The typical plant was found in the Arctic region, on the 

 island of Spitzbergen. It is described by Dr. Nordstedt as 

 measuring 48-51 ,. in diameter, about one-fifth larger than 

 our plant ; hitherto found only in one locality, 1S81 and 1882. 



