

167 
6, and especially that of September 27, when they were found con- 
tinuously for a month. 
The separate records of the type and the variety (Table I.) contain 
in their seasonal distribution one point of special interest; namely, 
the appearance of the variety after the type has been present for 
some time. An examination of the records in the several years 
reveals the fact that var. bzdens is practically confined so far as 
large numbers are concerned to the months of July-September. This 
appears in 1898 (Table I.) and is equally evident in 1896 and 1897, 
but is less noticeable in 1895. The first large pulse is passed in each 
year before var. bidens takes any appreciable part in the genesis of 
the pulses. Even the second large pulse is not extensively con- 
tributed to by the variety in some instances. On the other hand, 
the later pulses in 1895 and 1897 were mainly of the variety. There 
is thus in this species some evidence of a tendency on the part of the 
variety marked by the development of a pair of posterior spines to appear 
an the latter part of the pertod of seasonal occurrence. 
The variety bzdens in our records includes individuals with well- 
developed spines (6. caudatus Barrois and v. Daday), but they are 
not to my mind worthy even of varietal distinction, since they 
intergrade so completely with var. bidens and are merely well-de- 
veloped examples of this variety, and I see no reason for giving the 
variety two names. 
Wesenberg-Lund (00) has expressed the opinion that the elonga- 
tion of structural processes which he has noted in summer planktonts 
is an adaptation on their part to the changes in the buoyancy of the 
water dependent upon changes in its specific gravity and, as shown 
by Ostwald (03 and’03a), in its molecular friction caused by seasonal 
fluctuations in temperature. It would seem that this tendency on 
the part of the spinous form of Brachionus angularis to appear in 
greater proportions in late summer at the period of maximum heat 
in our waters might be an illustration of Lund’s thesis and Ostwald’s 
theoretical considerations. The changes in temperature during the 
occurrence of the species are, however, not very great, though our 
incomplete records suggest (Pt. I., Table III. and Pl. X.—XII.) that 
August temperatures are higher on an average than those of July. 
The averages for June, July, and August are 77.75°, 81.03°, and 
81.49°. In 1897, the dominance of the spinous type extends well 
into September, but it accompanies a period of summer heat (Pt. L., 

