102 FRESH- WATER RHIZOPODS OF NORTH AMERICA. 



by a narrow constriction, as seen in figs. 2-9, pi. XII. I think there can 

 be but little doubt that this variety and the ordinary more familiar form of 

 D. piriformis merge into one another ; and I have met with various inter- 

 mediate forms. See figs. 24-26, pi. XIX. 



The specimens vary greatly in size, and this is also the case in the 

 proportions of the body and neck of the shell. Ordinarily those from 

 Absecom pond range from ifh to ith of an inch in length. The smallest 

 specimen observed, from Swarthmore brick-pond, was ith of an inch long 

 and ith of an inch broad. The largest one, from Absecom pond, was 

 55th of an inch long by ^th of an inch broad. 



Still another variety which I have occasionally seen is represented in 

 figs. 17, 18, pi. XII. It may be distinguished as B. cornuta, and has the 

 characteristic form of D. pyriformis, but has the fundus provided with one 

 or two conical spines. This variety I have observed to approximate by 

 intermediate forms the more characteristic ones of D. acuminata. 



The shell of Difflugia pyriformis is commonly composed of coarse, 

 irregular grains of hyaline quartz-sand, and is often the roughest of its 

 kind; though sometimes, considering the coarseness of the materials, the 

 shell is wonderful for its evenness. The shell may be composed altogether 

 of comparatively large stones, or it may be partly constructed of these 

 more or less uniformly distributed, with the intervals filled in with smaller 

 ones. Sometimes the larger stones especially occupy the fundus or the 

 greater part of the body, and the neck is composed of smaller ones; some- 

 times the arrangement is reversed, and sometimes the largest stones are 

 arranged contiguous to the mouth. Not unfrequently one or several stones 

 greatly exceed the others, and produce conspicuous unsymmetrical projec- 

 tions in the shell, as seen in figs. 6-9, 12-15, pi. X; figs. 37, 38, pi. XVI. 



It may not be unworthy of remark, that the coarsest and most uneven 

 specimens are found together in association with the smoothest, in the 

 same localities and apparently under the same circumstances. 



Less frequently the shell of Difflugia pyriformis is constructed of an 

 intermixture of quartz-sand and diatoms in varying proportions, usually 

 the former predominating, but sometimes the latter. Large naviculas are 

 often conspicuous among the building materials, and occasionally spicules 

 of the fresh-water sponge are noticed with them. Such specimens are 

 represented in figs. 13, 15, 18-21, pi. X. Sometimes the shell of D. pyri- 



