95 



fish (Forbes, '80) such as the Catostomidce and some of the Silu- 

 rida and minnows. I have found them in great abundance in the 

 intestine of the adult gizzard-shad (Dorosoma), and in the contents 

 of the digestive tract of the German carp (Cyprinus carpio). 



In the pages which follow, the seasonal distribution, or occur- 

 rence in the plankton, of thirty-one Rhizopoda is discussed, and 

 the presence in the plankton of the Illinois of twenty-eight other 

 rhizopodan forms which have been recognized by other writers as 

 of specific rank is noted. This by no means exhausts the rhizopo- 

 dan fauna of the environment which was the field of this investiga- 

 tion. A continued study of the plankton itself would doubtless 

 greatly extend the list of adventitious forms from the shore and 

 bottom, and a more careful analysis of the variants, especially in 

 the Difflugia globulosa-lobostoma group, would still further increase 

 the richness of the fauna from the systematic point of view. Hem- 

 pel ('99) lists sixteen species from this locality, and Penard ('02), 

 in discussion, remarks: "Une pareille pauvrete dans une region 

 riche en organismes de toute nature, est une impossibilitie mat 6- 

 rielle." However, neither Hempel's paper nor the present one 

 pretends to give a full account of all the Rhizopoda of the region. 

 He dealt largely with plankton collections, and the present paper 

 deals with them exclusively. 



There is but little in plankton literature which gives with any 

 fulness the seasonal distribution of the Rhizopoda, or indicates that 

 they are of any considerable importance in the economy of the 

 plankton. The importance which they acquired in the plankton of 

 the Illinois is no doubt in part due to the nature of the environ- 

 ment with which we are dealing. The somewhat sporadic and 

 meteoric character of their appearances in our waters leads to the 

 inference that full seasonal analyses of the plankton of other bodies 

 of water at brief intervals may reveal a greater prevalence of the 

 Rhizopoda in the plankton than has hitherto been detected. 



DISCUSSION OF SPECIES OF RHIZOPODA. 



Amoeba Umax Duj. This was frequently abundant in the water- 

 bloom of midsummer, but was not identified in the plankton 

 collections. 



Amoeba proteus Rosel. Average number, 342. The individuals 

 here assigned to A. proteus include those taken in our plank- 



