Ill 



forms their margin. Chitinous, brownish, or more or less trans- 

 parent shells are abundant when pulses occur. Forms which 

 connect this species with D. globulosa have been observed. In- 

 cluded with D. lobostoma are forms which have since been described 

 by Penard ('02) as D. gramen, D. gramen var. acETom, and D. 

 lithopUtes, though I have not found in the Illinois plankton any of 

 the last-named with the peculiar tipped horns found by Penard 

 upon many individuals of his species. 



Difflugia pristis Penard (?). A small Difflugia was found occa- 

 sionally in the filter-paper collections in the colder months, but 

 only from November to March. It was often dark, or even blackish, 

 resembling in this respect Penard's D. pristis. Individuals not 

 thus darkened approach more nearly D. fallax Penard and D. puleoc 

 Penard. 



Difflugia pyriformis Perty. Average number, 368. This species 

 occurred in every month except January, but generally in small 

 numbers and irregularly. The largest number taken 12,000, on 

 May 25, 1896 came with the flood at that time (Pt. I., PI. X.), and 

 all the large occurrences of 1898 came with rapidly rising water 

 (cf. Table I: and Pt. I., PI. XII.). There are no indications of pulses 

 during stable conditions, and we must conclude that the species is 

 purely adventitious in our plankton. It is one of the largest species 

 with a heavy shell, and its flotation is impeded thereby. 



This species is exceedingly variable. The following varieties or 

 variants, given specific rank by some writers, have been noted, and 

 are included with D. pyriformis in the enumeration: D. pyriformis 

 var. nodosa Leidy, D. pyriformis var. daviformis Penard, D. pyriformis 

 var. venusta Penard, and D. pyriformis var. lacustris Penard. A 

 more slender and smoothly contoured form than the last is not 

 uncommon. 



D. capreolata Penard and D. bacillifera Penard were also found , 

 but are rare. 



Difflugia rubescens Penard was taken but once on May 25, 1896. 



Difflugia tuberculosa Hempel was also found but once in the 

 planktons enumerated, though Hempel ('99) reports it as appearing 

 occasionally from August to November in 1895. 



Difflugia urceolata Carter was taken only in April and May, 1896, 

 in small numbers at temperatures of 66- 80. 



