412 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxv. j 



terior and posterior ends evenly rounded and tuberculate. Anterior 

 end slightly wider than the posterior, and tubercles more numerous. 



Seen from above (fig. 3), the shell is elongate, sides nearly parallel, 

 anterior end pointed, with left shell overlapping right ; posterior 

 margin rather bluntly rounded. The characteristic protuberances 

 and furrows of the shell in the region of its anterior third are not so 

 decided as in /. gibba. I . gibba has, in addition, a protuberance on 

 each shell just posterior to its middle, which is larger than the others 

 and characterizes the species; several other small tubercles at the 

 anterior and posterior margins may also be plainly seen from above. 



Natatory setae of the second antennae reach but to the tips of the 

 terminal claw.-. 



Terminal segment of second foot (fig. 5) conical, five-eighths as 

 broad as long and bearing three seta 3 , the two terminal backwardly 

 directed, the shorter being about one-half the length of the longer, 

 the longer being approximately seven times the length of the terminal 

 segment. A third seta is present which is commonly pointed in the j 

 same direction as the two terminal ones, thus causing an appearance 

 of three terminal backwardly directed setae. This seta is situated 

 laterally rather than terminally, approximately near the middle of the 

 segment, and is nearly as long as the longer of the two terminal setae. . 

 Inner margins of penultimate segment of second foot with two 

 long seta?, of which the distal one is the longer, the shorter one being 

 two-thirds its length, or about the length of the shorter terminal seta. 



Furca (fig. 6) strong, curved, much broadened at base, and ap- 

 proximately ten times as long as width at middle. The two terminal 

 claws slender, plain, approximately same length, and seven-twelfths 

 as long as furca; terminal seta very weak, about as long as average 

 of furca. Dorsal seta situated at one-third length of furca from tip, 

 slender, plumose, bent near tip, and reaching about three-fourths 

 distance to tip of furca. Distal half of dorsal part of furca ciliate, 

 also sparsely ciliate laterally. 



This species is not free swimming, but creeps or burrows. It 

 appears to be closely similar to /. gibba, var. repens, of Yavra." 

 excepting that the natatory seta 1 are longer than is the case with 



Described from a number of specimens kindly sent me by Prof. 

 A. E. Beardsley, of the State Normal School, Greeley, Colorado. 

 Professor Beardsley collected them from Carters Slough, near Cree- 

 ley, Colorado, March 9, 1905. He has already made note of its 

 occurrence. '' 



Distribution. — Europe (Sars, Miiller, and Kaufmann), Britain 

 (Brady and Norman), Colorado. 



"Arch. Naturw. Durchforsch. Bohmen, VIII, 1891, p. 60. 



6 Science, new sei\, XXI, pp. HisT-OSS. 



