17G FRESHWATER RHIZOPODS OF NORTH AMERICA. 



prominent sides of the body of the shell. Viewed from the side (figs. 2, 4, 

 6, 8, 9, 11), it is variable in shape, though ordinarily it is more or less 

 balloon-like or rounded mitriform, commonly of greater height than the 

 breadth, and narrowed at or near the base. 



The dome is mostly inflated, and is round and even, or both it and the 

 sides may be depressed into a variable number of faces defined by folds. 

 The base is circular, and sometimes irregularly and widely dentate in out- 

 line, and it is more or less deeply inverted in a funnel-like manner toward 

 the mouth. The latter is circular, with a variably crenulated border, and 

 is everted into the top of the inverted funnel-like base of the shell. 



The color of the shell of A. mitrata presents all the shades of difference 

 occurring in A. vulgaris. 



The sarcode, as in the latter, is colorless. It usually forms an oblate 

 spheroidal mass connected with the mouth by a cylindroid neck of variable 

 length. Sometimes the sarcode mass is prolate spheroidal, and often demi- 

 spheroidal or demioval, in which case the base mostly rests upon the posi- 

 tion of the mouth of the shell. The body of the sarcode is ordinarily 

 attached by a multitude of diverging threads of ectosarc to all parts of the 

 interior of the shell. 



Pseudopods appear commonly to be more numerous in A. mitrata than 

 in the other forms, but in other respects do not differ. 



Generally, the size of A. mitrata exceeds that of Arcella vulgaris. It 

 ranges in height from about ^th to ^th of an in(h; in breadth at base 

 from i,th to ^.th of an inch; in breadth near the middle from ~t\\ to r J r th 



SoO 150 300 1J5 



of an inch. The mouth ranges in width from ^- th to ^ th of an inch, and 

 its elevation above the base, exclusivo of the eversicn, is from ^th to j^th 

 of an inch. 



I am uncertain whether a form like that of Arcella mitrata has been 

 previously recognized. I had suspected that it might be the same as 

 Arcella costata of Ehrenberg, but this is doubtful from the absence of ribs, 

 and its being more than three times the size. Since first describing it, in 

 looking up the literature of the rhizopods, I met with Mr. Archer's descrip- 

 tion of Arcella globosa, with which it appears best to agree, and perhaps it 

 may be the same. 



