84 
tion, structure, and sporulation thus suggest the possibility that 
Mallomonas is a free zodid stage leading to sporulation in Synura, 
they do not demonstrate it, and the genera must stand 7m statu quo 
until breeding experiments shall clearly demonstrate the full life- 
cycle of Synura. 
Pandorina morum Bory.—Average number, 6,957. In 1898 this 
organism was about half as abundant as Eudorina, but in 1897 it 
more than equals it. On account of the small size and the motility . 
of the colonies many of them escape through the silk, so that it is 
not so adequately represented in silk-net collections as Eudorina. 
It is probably the most important quantitatively of the Volvocide 
in our plankton. It occurs from April to October, with a few spo- 
radic appearances in March and up to January. Its greatest growth 
occurs from May to October. There is no predominant vernal pulse 
in 1898, but a series of smaller ones culminating May 3 (48,400), 
June 14 (60,000), July 26 (63,200), and August 30 (3,200),—all upon 
declining floods (Pt..I., Pl. XII.) and-coincident with pulses of 
other Volvocide—Eudorina and Gonium. In 1897 its seasonal dis- 
. tribution was also similar to that of these genera, exhibiting a max- 
imum pulse August 31 of 638,000 at 80°. In 1896, a year of inter- 
rupted hydrograph (Pt. I., Pl. X.), Pandorina attained no marked 
development. Its optimum temperatures lie at and above 60°, 
and its larger pulses appear during the season of maximum temper- 
ature, that is, at about 80°. Pandorina does not attain any marked 
autumnal growth, but declines in September, and as a rule dis- 
appears in October. The period of its growth thus lies within that 
of the land flora. 
As in Eudorina, so also here, parasitism by Dangeardia mammil- 
lata is of frequent occurrence. Pandorina is an important element 
in the food of summer rotifers such as Brachionus. 
Peridinium tabulatum Ehrbg.*—Average number, 3,875,769; 
silk, 3,711. This is a perennial planktont, having been found in 
every month of the year. Its principal development is, however, 
reached during warmer months, from May till September. In 1897 
the maximum pulse of 172,800 was on August 10,and in 1898 one of 
66,800 fell on July 26, the temperatures being 81° and 89° respectively. 
The only exception to this predominance in warm months is an iso- 
lated pulse of 2,400 which developed on the declining flood of Febru- 
ary, 1899 (Pt.I., Pl. XIII). Theabsenceofany autumnal development 
