North American Fresh-water Cyclopidce. 47 



The receptacidum seminis (PI. XII., Fig. 4) is regular in 

 outline, the anterior border being a low arch extending com- 

 pletely across the segment. The posterior portion is much 

 deeper and about half as wide. The broad spermal canals 

 arise from the anterior angles. The porus is exactly between 

 the two anterior angles. 



The egg-sacs are usually small and round in the specimens 

 from large lakes ; otherwise they are elliptical and very large. 



The usual length of the female is 1-1.4 mm. 



This species is very widely distributed in America. It has 

 been found in Massachusetts and Wyoming and in all the 

 intervening territory. It is the common pelagic species of the 

 Great Lakes, but also occurs in large numbers in our ponds 

 and rivers. Inhabiting, as it does, such a great amount of 

 territory and such a variety of situations, it is not strange 

 that it proves to be a very variable form. The Massachusetts 

 form is exactly as described by Dr. Schmeil. In Wyoming 

 this species is very much more slender in all its details, 

 though not differing markedly in any other way from the type. 

 Between these two extremes in location are a great number 

 of intermediates in form. The fifth foot is rather variable, 

 but I note no variation in the shape of the receptacidum semi- 

 nis. The vertical combs of spines on the stylets are always 

 present, but are not always conspicuous in the Western speci- 

 mens. Although the most attenuate specimens examined 

 were usually from large lakes and the most robust from ponds, 

 I find that no other generalizations are possible with regard 

 to the character of the specimens in connection with the 

 nature of the situation — probably on account of the frequent 

 transfers of individuals from one of these situations to another. 



Subgenus Macrocyclops Claus. 

 Cyclops albidus Jurine. (PI. XIII.) 



Monoculusquadricornis var. albidus, Jurine.'20, p. 44. PI. II.; Fig. 10,11. 

 Cyclops gyrinus, Forbes, '90a, pp. 707-709, PI. II., Fig. 9; III.. Fig. 14. 

 Cyclops signatus var.tr mi icoi-tiis^Herrick and Turner, "95. pp. 106.107, 



PI. XV.. Fig. 5-7: XX.. Fig. 1-7; XXXIII., Fig. 1,2. 

 Cyclops albidus, Schmeil, '92, pp. 128-132, PI. I., Fig. 8-14b; PI. IV., 



Fig. 15. 



