DIFFLUGIA OBLONGA. 13 



lacitstris but is smaller and free from sand-grains, 

 the test being thin and transparent. 



The var. lacustris is not infrequently found in conju- 

 gation during the summer months. In Cheshire ponds 

 it is frequently associated with the var. venusta Penard 

 and with D. lanceolata Penard as well as with typical 

 D. oblonga Bhrenb. and D. acuminata Bhrenb. Lacus- 

 trine forms described by Penard as occurring in 

 Switzerland are more robust than our own ; though 

 an example from the Chipperfield pond was found to 

 measure 260 /a. 



Fig. 40. — Biffiucjia oblonga var. lacustris in conjugation, x 200. 



Biffliigia oblonga var. nodosa {I), pyriformis var. 

 nodosa Leidy, ' Freshw. Rhiz. N. Amer.,' p. 99, t. x, 

 ft. 7-22) was originally described by Leidy as a distinct 

 species under the name of Diffiugia entochloris, but 

 afterwards was admitted by him to be not specifically 

 distinct from D. pyriformis {oblonga Ehrenb.). The 

 test is more or less compressed, and is distin- 

 guished from other varieties by its broad and uneven 

 crown, which is produced into "three knobs, or conical 

 eminences, varying greatly in degree of development." 

 It is found in ponds, according to Leidy, usually asso- 

 ciated with the type, but so far as our observation 

 extends, it is non-existent in this country. At first 

 we were inclined to associate D. crassa sp. nov., from 

 Chipperfield, with it, but a comparison of the figures 

 shows that they are distinct. 



2. Difflugia crassa sj). nov. 

 (Plate XVIII, figs. 1-3.) 

 Test large, composed of coarse sand-grains, dense 

 and opaque ; compressed, and very broad proportion- 



