98 
individuals which present intermediate characters. The majority 
of the individuals were taken in a living condition, though many 
empty shells were found. The conditions of the examination of the 
plankton and the opacity of many of the shells made it impossible 
to distinguish the dead shells in all cases. The records include many 
dead shells. 
Arcella costata Ehrbg.—Average number, 48. For the purposes 
of this paper I have included here all those individuals which possess 
an angular or ribbed shell. , Leidy (79) refers such formsetammns 
vulgaris. Individuals of this type are rare, occurring infrequently 
and in small numbers. It was recorded but 18 times in thesso0 
collections, and the largest number per cubic meter was only 1,187. 
As in the other species of the genus, the warmer months are favored, 
fourteen occurrences falling in June-September in water at 70° or 
above. The other four records are one each in April, October, 
November, and December. The seasonal range of this form in the 
plankton thus falls in the main within the period of the maximum 
abundance of A. vulgaris, of which species it may be but a variant. 
Arcella discoides Ehrbg.—Average number, 972. This prevalent 
species is not in all instances easily separated from A. vulgarts. 
Indeed, even Leidy (79) states that it graduates into A. vulgaris, 
and that he views it as the variety of this species in ‘‘which the shell 
presents a greater proportionate reduction in height compared with 
the breadth.’”” In the enumeration of our plankton catches, the 
larger, flatter, and unornamented individuals have been referred to 
this species. Both the brownish and the hyaline forms should 
probably, for reasons hereafter given, be included here, and they 
are so grouped in the present discussion. Thus considered, A. 
discoides is the most abundant member of the river plankton be- 
longing to this genus, including two thirds of all the individuals 
observed. 
This species occurred in almost two thirds of the collections, hav- 
ing been recorded in 115 of the 180, and more frequently and in 
larger numbers in the latter half of the five years than it was in the 
earlier period. This is in part explained by the unusual fluctua- 
tions of the river levels in 1898, during the maximum summer 
occurrence of the species. Like the other species of the genus, A. 
discoides has a period of maximum occurrence in the latter part of 
summer, as is shown in Table I. Of the 115 occurrences, 55 were in 
