1914.] NATURAL SCIENCES OF PHILADELPHIA. 35 



of the last thoracic segment. The second and third segments are 

 cylindrical and their posterior margins are very slightly and unevenly 

 serrated, largely on the under side. The fourth segment has the 

 usual fringe of spines on its posterior edge, and tapers suddenly to 

 the insertion point of the stylets. The stylets (PI. I, fig. 4) are 

 short, but slightly divergent and smooth on their inner sides. This 

 is an important character which Forbes does not note in his descrip- 

 tion. The proportion of the length to the breadth of each ramus 

 is 3 : 1 . There are four well-developed apical bristles. The longest 

 is to the second in length as 7 : 5; the outer to the inner as 1 : 3. 

 They are all plumose, but not as densely so as in Cyclops fuscus 

 J urine. 



The first antennae vary very little in length. In the female they 

 usually reach to the middle or posterior border of the last thoracic 

 segment. They are seventeen-jointed, the terminal joints attenuated, 

 the last three being each armed with an hyaline plate. The edges 

 of these plates on the fifteenth and sixteenth segments are for the 

 most part entire, Ijut I have-repeatedly seen them, especially at the 

 base of the fifteenth segmental plate, minutely serrated. More 

 rarely these serrations extend along the entire edges of all three 

 plates. The plate of the last segment is always finely serrated on 

 its distal half. The point where these serrations cease and the 

 smooth edge begins is sharply defined by a much deeper notch or 

 indentation (see PI. I, fig. 8). The twelfth segment bears a well- 

 developed sense-club (PI. I, fig. 10). Its length is about equal to 

 that of the thirteenth segment. All of the segments, except the 

 three terminal ones, bear an irregularl}^ broken, longitudinal row of 

 minute spinules on their under side. The eighth, ninth, tenth, 

 twelfth and thirteenth segments have each a short row of small 

 cone-shaped serrations at the point of juncture with the following 

 segments, as in Cyclops fuscus. The twelfth segment (PI. I, fig. 10) 

 has, in addition, several (usually two) rows of smaller spinules 

 extending parallel to the marginal semicircular row. 



Marsh ('95) failed to find these "crowns of spines" on the antennae 

 of "a large number of mature females" of C. albidus which he 

 "examined with great care." He concludes that this peculiar 

 character "seems to be rarely true in our forms." Forbes has 

 found it in the specimens examined by him from many parts of the 

 country. I have never failed to find it in the local specimens. 



The third segment of the second antennae (PI. I, fig. 6) is short; 

 and somewhat pear-shaped. The armature of the swimming feet is- 

 as follows: 



