61 



blage of animals associated in the nodule-bearing beds of Mazon Co. 

 in the same State. 



The penultimate and antepenultimate segments are of nearly equal 

 breadth, the former nearly as long as broad, and quadrate; the latter 

 is more than half as long again as broad, and also quadrate ; the last 

 segment is not preserved, but by the relation of the surrounding 

 parts appears to have been triangular and nearly equiangular; at- 

 tached to the outer sides of this segment are the caudal appendages, 

 which diverge at a small angle; these appendages are slightly longer 

 than the three terminal segments of the abdomen, straight, depressed, 

 lamellate, tapering regularly beyond the middle and longitudinally 

 sulcate throughout, as well as finely and obscurely striate near the 

 tip ; the extremities of both are broken, but were apparently pro- 

 duced to a fine point. There is no median spine, nor, to judge from 

 the relation of the lateral spines at their base, and by comparison 

 with the same parts in the trebly spined genera Ceratiocaris and 

 Dithyrocaris, did one ever exist; if really bicaudate, this genus differs 

 distinctly from any we know. 



The most striking and interesting feature in this crustacean, how- 

 ever, lies in the nature and distribution of the lines of ornamenta- 

 tion upon the dorsal surface of the abdominal joints. So far as I 

 am aware, the striation of the abdominal joints of these low and 

 ancient Branchiopoda has always hitherto been found to take the 

 form of imbricated lamellse, and the lines thus formed run parallel, or 

 nearly parallel, to one another. In Rhachura,^ as the fossil from Illi- 

 nois may be called, this is not the case, the markings being ordinary 

 raised ridges, or, if in reverse, they appear as impressed lines or 

 furrows, which branch more or less from one another. Their dis- 

 tribution on the antepenultimate segment is most remarkable; with 

 the exception of one or two short and feeble lines next the outer 

 edge, which run obliquely forward and parallel to each other, they 

 either converge by running in a curving course toward the anterior 

 outer embossed angles of the segment, or they join others which do 

 so. At this angle, the principal lines, or those either made up of 

 the union of several branches, or running independently to this point, 

 do not quite meet, but lie side by side, just as do the principal veins 

 of an insect's wing ; so that these principal lines, with or without 

 their branches, spreading in all directions over the half of the seg- 

 ment belonging to them, imitate, to an extraordinary degree, the 

 branching or simjile veins of an insect's wing. Moreover, just as in 



1 Derived from'paxos, oiipa. 



