104 



importance in its economy. It includes a number of forms notorious 

 for their variability and for the difficulty with which specific dis- 

 tinctions can be applied. I shall discuss the species as they were 

 enumerated, and shall correlate my work with Penard's ('02) recent 

 elaborate analysis of the species so far as I can with the aid of my 

 notes in the absence of the collections. Opinion as to the validity 

 of the species is expressly withheld excepting in those instances in 

 which it is formally stated. 



Difflugia acuminata Ehrbg. Average number, 315. This spe- 

 cies has occurred in every month of the year and in 83 out of 180 

 collections. In 1898, two thirds of the occurrences and three fourths 

 of the individuals were taken between May 1 and October 30, at 

 temperatures above 70. In this year there are six recurrent pulses 

 from June to November, but all but one of these are found on rapidly 

 rising flood waters, and they bear no constant relation to the pulses 

 of diatoms previously noted, with which in some instances they 

 are intercalated, though this is not regular or constant. Similar 

 tendencies to appear with floods and in greater numbers and more 

 frequently in summer can be detected in records of other years. It 

 was more than twice as abundant in 1896 a year of interrupted 

 hydrograph (Pt. I., PI. X.) as in 1898. This is one of the larger 

 and heavier rhizopods, and its occurrence in the plankton is doubt- 

 less adventitious, due to floods and currents, and its greater numbers 

 and frequency in the summer may result from its greater abundance 

 at that season in its natural habitat, the shore and bottom, and 

 perhaps, also, from its lighter specific gravity during the warmer 

 season. An illustration of this appears on the rising flood of June, 

 1897, when the maximum number recorded (10,000 per m. 3 ) oc- 

 curred. 



The shell of this species is exceedingly variable in size, constitu- 

 ent particles, and proportions. A number of forms separated by 

 Penard ('02) and others as distinct species were grouped under D. 

 acuminata in the enumeration. The greater number of these belong 

 to the type designated by this name by Penard ('02). D. acuminata 

 var. inflata Penard and the somewhat similar D. elegans Penard are 

 not uncommon. D. acuminata var. umbilicata Penard, D. elegans 

 var. teres Penard, D. curmcaulis Penard, D. lance olata Penard, and 

 D. scalpellum Penard occur also, but are rare. 



