400 PALAEONTOLOGY OF ILLINOIS. 



the radical difference from all the known types of that extensive group, of 

 having only one pair of the abdominal appendages, styliform, and the other 

 five natatory, instead of three pairs styliform and three natatory; thus combin- 

 ing with its Amphidodan abdomen, thorax, head, anterior appendages, and 

 general physiognomy, the single pair of styliform appendages of the Isopoda. 



Being therefore left in doubt in regard to its affinities, we sent sketches of 

 some of the best specimens to Prof. Dana, who had also previously received 

 some imperfect specimens of the same species from Illinois. On examining 

 these sketches and specimens, Prof. Dana wrote that he thinks this crustacean 

 most probably belongs to a group holding an intermediate position between the 

 typical Isopoda and the Amphipoda, for which he has proposed the name Aniso- 

 poda. This intermediate group, as first shown by Prof. Dana, is characterized 

 like the Amphipoda, by having the three posterior pairs of thoracic legs in one 

 series, and the four anterior in another; while, as in the Isopoda, the branchiae 

 are abdominal, and only one pair of abdominal appendages are styliform, and 

 five branchial. 



In regard to the division of the thoracic legs into two series, we would 

 remark, that we have observed no evidence of it in all the specimens we have 

 seen, excepting one of those kindly loaned us by Prof. Dana. In this, however, 

 four pairs of these legs seem to be directed backwards, and only three forward; 

 which, if not produced by accidental distortion, would indicate Isopod affinities. 

 Yet, in all the other specimens seen, the whole seven pairs are directed forward. 

 Although much inclined to believe the latter their normal arrangement, it 

 should be remembered, as suggested by Prof. Dana, that however important 

 this character may be in the study of the recent Tetradecapod Crustacea, it can 

 scarcely be made available in the investigation of crushed fossil species, where 

 so many accidents might have occurred to place the legs in an unnatural 

 posture. 



We have not been able to clearly satisfy ourselves whether or not our crusta- 

 cean had squamiformly developed epimerals, as in the normal groups of 

 Amphipoda ; though some of the specimens appear to show indications of such 

 development; while the shortening of the vertical diameter of the thoracic 

 segments, as compared with those of the abdomen, would seem to be, as it were, 

 an arrangement to make room for such scale-like epimerals. In addition to 

 this, the fact that all of the six or eight specimens we have yet seen, lie in the 

 concretions upon one side or the other, would appear to indicate that the lateral 

 motion of the thoracic legs was in some way restrained so as to prevent the 

 animal from taking an erect posture, which is precisely the effect produced in 

 the normal Amphipoda by the possession of well developed squamose epimerals. 

 If this should prove to be the case, it would show that the remarkable combi- 

 nation of Amphipod and Isopod, or Anisopod characters, already alluded to in 



