.402 PALEONTOLOGY OF ILLINOIS. 



First pair of thoracic legs about one-fourth longer and a 

 little stouter than the succeeding five pairs, and apparently 

 terminating in a slender, sharp dactylus ; first joint above a 

 little shorter, narrower, and more tapering than the next — 

 neither more enlarged than the others above. Five suc- 

 ceeding pairs of legs of nearly equal size and form; their 

 upper two (or three?) segments very short, and not enlarged; 

 seventh pair nearly as long as the first, and more slender 

 than the others. Natatory abdominal appendages long and 

 slender; styliform pair with first segment short and quadrangu- 

 lar ; second and only other joint (double?) with each branch 

 (if there are two) simple, equal and as long as the telson, 

 which they nearly exactly resemble in form ; their upper and 

 lower margins each with a row of short, oblique, rather distant 

 setae, between which may be seen, by the aid of a magnifier, a 

 series of much more minute, closely-arranged setse. Telson as 

 long as the last four abdominal segments ; at its base one-half 

 as wide vertically as the penultimate abdominal segment ; 

 thence tapering, at first rapidly and then very gradually, to a 

 mucronate point — upper and lower margins setigerous, like 

 those of the stylets. 



It is possible that when we can have an opportunity to examine additional 

 specimens, we may have to modify some of the characters given in the forego- 

 ing generic or specific description; though not, we believe, in any very essential 

 particular. We hope, however, to be able hereafter to add other characters, 

 and to clear up several doubtful points in the structure of this interesting type, 

 when we can have better specimens for study. 



Length from anterior side of head to the extremity of the penultimate abdo- 

 minal segment, 1.30 inches; length of telson, 0.31 inch; length of the first six 

 abdominal segments, 0.52 inch; length of the seven thoracic segments, about 

 0.64 inch. Height of third abdominal segment, 0.20 inch; height of each first 

 two or three thoracic segments, 0.12 inch; length of stylets about 0.31 inch. 



Locality and position : Same as preceding. 



