100 FRESH- WATER RHIZOPODS OF NORTH AMERICA. 



Difflugia pyriformis is one of the most common species, and it 

 presents much variety of shape and size. 



The shell is ordinarily flask-like or baloon-form, or, as indicated by 

 the specific name, pear-shaped, with an oval or ovoid body more or less 

 gradually prolonged into a neck, which tapers to the mouth or is cylindroid, 

 and of variable proportionate length. Usually the shell is of uniform 

 diameters, but is sometimes more or less compressed so as to be wider in 

 one direction. Occasionally specimens occur exhibiting some want of 

 bilateral symmetry. The fundus of the shell is mostly regularly rounded, 

 but sometimes is more or less subacute. The mouth is inferior, terminal, 

 and circular or oval. 



Characteristic specimens of the ordinary forms, exhibiting considerable 

 variety in exact shape, size, and structure, are represented in pi. X. 



Specimens of Difflugia pyriformis of the ordinary kinds have a wide 

 range in size. Some of the smallest measure only the 4 -^th of an inch, 

 while large ones reach ten times that length. 



Compressed forms of D. pyriformis, constituting what I have viewed as 

 the variety — and agreeing with Mr. Carter's species — D. compressa, are 

 unfrequent compared with those with more uniform diameters. Interme- 

 diate conditions occur, showing the gradation of one form into the other. 

 In D. compressa, the shell exhibits variable degrees of compression, some- 

 times comparatively little, at others to such an extent that the shell is twice 

 the breadth in one direction that it is in the other. The length usually 

 exceeds the greater breadth, but rarely the latter actually exceeds the 

 former. Specimens exhibiting various degrees of relative breadth are 

 represented in figs. 10-16, pi. XII. 



The size of compressed specimens I have found to range as follows : 

 Length j|.th to ^th of an inch ; greater breadth ± th to ^th of an inch ; 

 lesser breadth ^th to Jgth of an inch ; with the oral end ^th to ^th of an 

 inch in the greater width. 



A striking variety of D. pyriformis, certainly not distinct from this as a 

 species, I have named D. nodosa. It is not an unfrequent associate with 

 the more ordinary form, and is distinguished by its usually large size, its 

 more or less compressed form, and its broad fundus, which is produced 

 into from one to three knobs or conical eminences, varying greatly in 

 degree of development. 



