90 DESMIDS OF THE UNITED STATES. 



truncate projection which in end vie-n- forms a rounded lobe ; 

 membrane rough with pearly granules which give a dentate 

 appearance to the margin. 

 Diameter 33-45 yw. 



K^ot uncommon ; probably no form is more generally dis- 

 tributed throughout the earth. It is variable in size, in the 

 arrangement of the central granules and in the projection. 



Tar. PEOTRACTUM. Plate LX, fig. 22 ; Plate XX, fig. 29. 



The figure (Plate LX) represents a form collected July, 

 1883, by my friend, Dr. Kitchel, in Lake Minnetonka, Minn. 

 He found it freely intermingled with the typical form (fig. 

 23 ). It is in all its parts, except the more protracted ends, 

 so like the true form, I conside)' it merely a variety of C. 

 ornatum. Plate XX, fig. 29, is a good representation of C. 

 protractum, Xaeg., but the specimens, found in large num- 

 bers, clearly show their connection with C. onwium. Com- 

 pare note under C protractum. 



C. Laoense. Xord. Plate XLIX, figs. 26, 27. 



Semi-cell in vertical view, in the middle much swollen 

 and the ends inflated, granules large; center smooth. It 

 is an exaggerated development of C. ornatum and hence a 

 variety of it. 



Tewksbury, Massachusetts, and not infrequently in New 

 Jersey ponds. 



C. Spoetella, Breb. Plate LX, figs. 28-30. 



Very nearly as long as broad, deeply constricted ; semi- 

 cells subreniform, sides somewhat dilated and rounded, end 

 broadly truncate, margins erose-spinous or denticulate; 

 cytioderm granular. • 



Diameter 33-36 ia. 



Harvey Lake, Luzerne County*, Pa. Bears some resem- 

 blance to C. ornatum, but ends do not protrude ; granules are 

 more spinous than rounded, producing the erose appearance 

 of the margins. Found many specimens in this one locality, 

 but did not recognize the form elsewhere. 



€. PEOTEACTUM, (Nacg. ), Archer. Plate XX, figs. 27-28. 



But slightly longer than wide, constriction deep, sinuses 

 narrow linear, often ampliated from the acute angles ; semi- 

 cells with tumid base, twice as broad as long, angles rounded, 

 sides deeply sinuate below the truncate end. Viewed from 

 the vertex, elliptic with a central inflation ; membrane 

 granular. 



Diameter 70-80 p.. 



