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. bidens 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" 1&9 6 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 

 in the latter part of the period of seasonal occurrence. 



The variety bidens in our records includes individuals with well- 

 developed spines (B. 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 

 byOstwald ('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. I., 



