1914,] NATURAL SCIENCES OF PHILADELPHIA. 23 



ramus of the fourth pair. The case of a spine becoming a seta as 

 m the outer ramus of the fourth foot of Number 3 right and Number 

 3 left is not as unusual as the appearance of an extra spine, as on the 

 outer ramus of the first pair of Number 3 compared with the cor- 

 responding ramus of 1 and 2. In some species the armature is 

 considerably more constant. In C. modestus, for example, it rarely 

 varies at all, specimens from widely separated localities showing an 

 identical arrangement of both spines and setse. After the examina- 

 tion of a great many specimens of a number of species, I have con- 

 cluded that unless there are very distinct differences in some of the 

 other important distinguishing characters, a slight variation in 

 swimming-feet armature is not sufficient proof for establishing a new 

 species or even a variety. 



The fifth foot, both in armature and shape, is constant as a rule. 

 There are slight variations occasionally in the shape of the segments 

 and comparative lengths of spines and setae. In a single case I have 

 found a mature female of C. viridis var. insectus with an extra com- 

 pletely developed seta on the distal segments of both fifth feet. 

 This very unusual form is now in the collection of C D. Marsh. 



The receptaculum seminis has already been mentioned. In pre- 

 .serving specimens it is advisable to use a 1 per cent, solution of 

 formalin to avoid contraction and distortion of this organ. 



All of the plates have been drawn from life. The movements of 

 the living animals may be readily overcome by the use of a 1-1000 

 parts solution of chlorotone. This I have found to be very con- 

 venient and all danger of flattening and distorting the outline of 

 specimens may be thus avoided. By this method the same individual 

 may be repeatedly used, as the animal recovers a few moments after 

 having been replaced in fresh water. Care must be taken that the 

 solution of chlorotone does not become concentrated by evaporation, 

 in which case the animals are killed by its too violent effects. 



The work in this paper covers a period of three years. All of the 

 species described herein have been taken from a single pond of less 

 than one acre area. The Copepoda having been largely neglected 

 in this part of the country, it is interesting to note the presence of 

 some of the more unusual forms, as C. varicans, C. fimbriatus var. 

 poppei, and several others. I am greatly indebted to Dr. C. D. 

 Marsh for his identification of specimens and his general interest and 

 assistance throughout the period of study. My thanks are also due 

 ]Mr. E. B. Forbes and Dr. H. S. Pratt. The latter, through his 

 unfailing personal interest and encouragement and by placing the 



