106 



an adventitious planktont. The data are too irregular to trace its 

 seasonal distribution. 



As a species it is as well defined as any in the genus. It is not in 

 our waters connected by intermediate forms with other species. Its 

 assignment to D. lobostoma by Schewiakoff ('93) is not in my opinion 

 justifiable unless we regard all forms of Difflugia as belonging to one 

 species. 



Difflugia fragosa Hempel. Average number, 25; in 1896 over 

 100. This species occurred in every month of the year but Febru- 

 ary, though three fifths of the records and the majority of the in- 

 dividuals were found between May and October at temperatures 

 above 60. The data are too irregular to trace the seasonal history 

 of the organism, but they suffice to suggest the agency of floods at 

 all times and of high temperatures during the summer, as factors 

 in the occurrence of the species in the plankton. The shell of this 

 form is relatively to that of other species rather heavy, and this fact 

 combined with the irregularity of its occurrence seems to justify 

 the conclusion that it is largely adventitious at all seasons of the 

 year. 



The species exhibits a great deal of variation in the development 

 of the central spine Hempel ('99, Fig. 1) and in the number and 

 arrangement of spines in the accessory circlet. The mammillate 

 form of the central spine figured by Hempel is not usually present. 

 Individuals in which the central spine is but feebly developed seem 

 to connect this species with D. varians, recently described by 

 Penard ('02). Otherwise, and in our waters, the species is well 

 delimited. 



Difflugia globulosa Duj. Average number. 7,194; in 1897, 

 47,329, the larger number in this year being in part due to a remark- 

 able pulse of 1,240,000 early in September. This is the most 

 abundant of all the rhizopods in our plankton, occurring most 

 frequently and in largest numbers. It is found in every month of 

 the year, and in 1898 appeared in every collection except four in De- 

 cember. With a few exceptions in the autumn of 1898 (Table I.), 

 no large development (exceeding 10,000 per m. 3 ) has taken place 

 earlier than May or later than September that is, at temperatures 

 below 60. The occurrences are most continuous and the numbers 

 of individuals are largest during the warmer period between the 

 months named. The largest pulse, that of 1,240,000 on September 



