122 FRESH- WATEE RHIZOPODS OF NORTH AMERICA. 



length, and wider from side to side than from before backward. In the 

 lateral view, with the plane of the mouth or bottom of the shell on a level, 

 it appears obliquely ovoid, with the fundus directed backward and upward, 

 and with the fore part of the base or anterior lip usually more or less 

 prominent. 



In the front or back view of the shell, as it is ordinarily seen, lying on 

 the object-plate of the microscope, by transmitted light, it appears pyriform, 

 ovoid or spheroid in outline, with a clearer transversely oval or somewhat 

 reniform or round space included within the lower or narrower part and 

 produced by the mouth. See figs. 2, 5, 7, 15, 22. 



The bottom of the shell is concave, and the nearly circular or oval 

 mouth is inflected and situated above the level of the border of the base. 

 The fundus is usually obtusely rounded and simple, and viewed from 

 behind is transversely oval and flattened below, as seen in fig. 13. 



In the largest and most elongated forms, the fundus is often provided 

 with from one to half a dozen acute, conical spines. A single spine pro- 

 duces a central, rather abruptly tapering point ; a pair surmount the sides, 

 and a greater number are ranged in a usually more or less regular row. 



Unsymmetrical forms of Difflugia constrida are not unfrequent, espe- 

 cially in the larger specimens, both in the shape of the shell and in the 

 arrangement of the spines, when these exist. 



The shell is ordinarily composed in the usual manner of other species 

 of the genus ; that is to say, of angular particles of quartz-sand. Sometimes 

 the particles have more or less uniformity ; sometimes heavier grains sur- 

 round the mouth, and not unfrequently also occupy the top of the fundus. 

 When spines are present they have the same composition as the body of 

 the shell ; but a remarkable circumstance is the frequent termination of these 

 spines with a single sharp-pointed and trenchant splinter, as if specially 

 selected for the purpose, and as represented in figs. 56, 57. 



Rarely the shell is composed of chitinoicl membrane incorporated with 

 variable proportions of scattered quartz particles, in the form of minute 

 grains or thin plates. Occasionally minute oval pellets, and sometimes dia- 

 toms, enter into the constitution of the shell. 



The sarcode of Difflugia constrida, independent of any food contents, 

 is transparent and colorless, and the animal is so very sensitive and indis- 

 posed to protrude its pseudopods, that in most cases it is difficult, in con- 



