148 FEESH-WATER EHIZOPODS OF NORTH AMERICA. 



the former, is composed of narrow rectangular plates, intermingled with a 

 comparatively few round or oval disks, as seen in figs. 13, 18, pi. XXII. 

 The narrow plates are usually placed parallel, in close juxtaposition, in small 

 patches, which are arranged in different directions, while the round or oval 

 plates occupy intervals here and there, singly or two or three together. 

 The general appearance gives the impression of certain kinds of tessellated 



pavement. 



Some shells are mainly composed of round and oval disks, of the 

 variety first described, intermingled with a few narrow rectangular plates 

 or rods, of various lengths, as represented in fig. 7. 



Occasionally I have seen specimens with extraneous matters, such as 

 quartz particles and diatoms, incorporated with the shell. A specimen 

 represented in fig. 8 appears to have a few sponge spicules added to the 

 ordinary intrinsic elements of composition. 



Not unfrequently there are found, in association with the usual more 

 characteristic varieties of Nebela coUaris, Individuals which have the same 

 form of shell, but with its structure rather related with that of the ordinary 

 forms of Difflugia. In some specimens the shell is composed of thin and 

 irregularly angular silicious plates, as represented in fig. 12, pi. XXIV. The 

 intervals of the plates appear as dark or clear outlines, according to the 

 focus, and the margin of the mouth of the shell is uneven, as it is formed 

 by the bordering plates. Mostly the intervals of the silicious plates are 

 conspicuous, and appear to be occupied by a clear cementing substance, 

 or the shell appears to be composed of clear chitinoid membrane paved with 

 the silicious plates, as seen in fig. 11. 



The specimens vary greatly in the forms of the component silicious 

 plates, which consist of variable proportions of the kind just described, 

 with others which are more regularly rectangular, or in the form of rods, 

 and sometimes with diatoms, and round or oval plates like those which 

 ordinarily compose the shell oi Nebela collaris. Through such specimens the 

 latter would appear by transition forms to merge into Difflugia compressa. 



The sarcode of Nebela collaris is colorless, but frequently the endosarc 

 appears more or less yellowish or brown, from the abundance of mingled 

 food in balls and as diffused granular matter. It is a remarkable fact that 

 rarely green food or food of any other color than those just mentioned is 

 seen in this animal. 



