46 PROCEEDINGS OF THE ACADEMY OP 



TETRADECAPODA. 

 ISOPODA. 



(ANISOPODA.) 

 ? Family ACANTHOTELSONIDJE. 

 Genus ACANTHOTELSON, Meek & Worthen. 



Superior antennae nearly as long as the inferior, and provided with well 

 developed accessory appendages ; flagella of both pairs longer than the pe- 

 duncles ; head about equalling the length of the first two anterior thoracic 

 segments. Thoracic and abdominal segments (except the last one) not dif- 

 fering materially in length, and each shorter than the head. Anterior thora- 

 cic legs longer than the others, not chelate. Telson or terminal segment 

 eimple, long, spiniform, and laterally compressed. Stylets with second seg- 

 ments (bifid?) much longer than the first, and similar to the telson. 



The fact that the oral apparatus, and other appendages of the head, as 

 well as the branchiae, always so important in the classification of the Crus- 

 tacea, are so rarely preserved in fossil species, especially those of smaller 

 sizes, renders their study more difficult than that of most other organic re- 

 mains. Hence, in describing new species, genera, or other groups, the palae- 

 ontologist is often unable to give any information in regard to the very parts 

 that would be the first to claim the attention of the Carcinologist in the in- 

 vestigation of recent species. Another difficulty also arises from the fact 

 that, as in other departments of palaeontology, it often happens in the study 

 of fossil Crustacea, we meet with types presenting a combinatiou of cha- 

 racters which in existing species are distributed in widely distinct groups. 

 So that on finding a new type showing only a part of its characters, we 

 would often be led at once to place it in a group with which probably the 

 next specimen found would show it to possess some one or more wholly in- 

 compatible peculiarities. 



On first examining specimens of the typical species of the genus above 

 described, our impression was, that it must be, beyond all doubt, a true Am- 

 phipod. A more careful examination, however, soon showed that it presented 

 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 com- 

 bining with its Amphipodan abdomen, thorax, head, anterior appendages, and 

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



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 crusta- 

 cean most probably belongs to a group holding an intermediate position be- 

 tween the typical hopoda and the Amphipoda, for which he has proposed the 

 name Anisopoda. 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 

 hopoda, the branchiae are abdominal, and only one pair of abdominal appen- 

 dages 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 evidences 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 indi- 

 cate Jsopod 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. 



[Mar, 



