228 FRESH-WATER RHIZOPODS OP NORTH AMERICA. 



pyriform reversed, but it is sometimes oval, ovoid, oblong, or more or less 

 constricted nearly on a line with the back part of the mouth. Sometimes 

 the oral extremity appears broader than the fundus, which is obtusely 

 rounded. The mouth is circular, and has a crenulate or beaded border. 



In the side view of the shell, in the position in which it is maintained 

 when the animal is in movement, the fundus is variably elevated. Some- 

 times the oral side of the shell is on the same plane as the mouth, and the 

 fundus is directed backward. In other specimens, the longitudinal axis 

 from the mouth to the fundus exhibits various degrees of inclination. The 

 oral end of the shell beneath is concave, or the mouth appears inverted, 

 forming the inner orifice of a shallow funnel. 



In many specimens of Trinema enchelys, especially in small and per- 

 haps young ones, the shell appears to be composed of clear homogeneous 

 membrane. In many others, especially large individuals, the shell is com- 

 posed of circular plates with beaded borders, conjoined by an intervening 

 amorphous cement, as represented in figs. 1-4. In some specimens, as seen 

 in tigs. 28, 29, the circular plates appear to overlap in the same manner as 

 in species of Euglypha. 



In many specimens of Trinema, especially those of intermediate and 

 of the larger size, in which the shell appears to be homogeneous, the out- 

 lines exhibit a more or less undulating character, apparently indicative of a 

 constitution of disks, though these are not perceptible. The interior soft 

 structure of Trinema is identical with that of Euglypha. Usually, two 

 contractile vesicles are observable at the sides of the nucleus, even in the 

 smallest individuals. The pseudopods are likewise of the same kind as 

 those of Euglypha. Three are commonly seen, whence the name of the 

 genus; but the number is by no means definitely restricted, and there may 

 be one to half a dozen or more. 



The size of Trinema presents a wide range The smallest one meas- 

 ured was 0.01 61 mm. long by 0.01 mm. broad, with the mouth 0.005 mm. 

 wide. The largest was 0.1 mm. long and 0.0625 mm. broad, with the 

 mouth 0.0238 mm. wide. Between these extremes I have observed all 

 gradations of size. 



As mentioned at the beginning of the present chapter, Trinema is one 

 of the most common of the Fresh-water Rhizopods, being found in almost 

 all positions in which other forms exist. A small form appears to be a pretty 



