144 



ERESH-WATER RH1ZOPODS OF NORTH AMERICA. 



Besides figs. 20-25, pi. XXIV, the accompanying figures, in the 

 arrangement of the plates of the shell, represent the more important varieties 

 which I have observed. 



The size of the shell ranges 

 from about the ^th to the ,4th of an 

 inch in length by ^.th to ^th of an 

 inch broad. 



The sarcode is colorless, and 

 in all its characters resembles that 

 of Hy alosphenia. The food- vacuoles 

 often appear yellowish. The nu- 

 cleus when visible measures about 

 the j^th of an inch or more. The 

 pseudopods are digitate and usu- 



Quadrula symmetrica. Opposite sides of the same shell. 



ally two or three in number. 



In several instances I have found specimens, late in the season, with 

 the sarcode in a quiescent or encysted condition. Fig. 25 represents such 

 a specimen obtained from sphagnum in November. It was of the largest 

 size, about the ~th of an inch in length. The plates were arranged ob- 

 liquely aci'oss the shell, and were pretty uniform in size. The neck was 

 occupied by a laminated diaphragm as thick as one third the length of the 

 shell. The sarcode mass formed a compressed oval ball occupying a cen- 

 tral position in the body of the latter The ball was nearly colorless or 

 faintly yellowish, and granular, and contained several large oil-like glob- 

 ules. A central clearer space appeared to indicate the presence of a nucleus. 



Dead shells are sometimes found containing in the interior a number 

 of scattered plates, or the same in one or more little packets, like those 

 composing the wall of the shell. 



Quadrula symmetrica was first described by Dr. Wallich, under the 

 name of Diffluyia symmetrica, from specimens found in England. It was 

 more recently described, and referred to a new genus, by Prof. Schulze, 

 from specimens found near Dresden. 



Ehrenberg described the same as pertaining to three different species, 

 under the names of Diffluyia assulata, D. carolinensis, and D. leptolcjns. These, 

 in 1871, with a number of other forms, he referred to a subdivision of 

 Diffiugia with the names of Assulina and Holoylyplia. As, however, the 



