INVERTEBRATES. 399 



TETRADECAPODA. 



ISOPODA. 



(anisopoda.) 



Genus ACANTHOTELSON, M. and W. 



Acanthotelson, Meek and Worthen, October, 1860. Proceed. Acad. Nat. Sci., Philad., 

 p. 457. 



Superior antennae nearly as long as the inferior, and provided 

 with well developed accessory appendages; flagella of both 

 pairs longer than the peduncles; head about equaling the 

 length of the first two anterior thoracic segments. Thoracic 

 and abdominal segments (excepting the last one) not differing 

 materially in length, and each shorter than the head. Ante- 

 rior thoracic legs longer than the others, not chelate?. Telson 

 or terminal segment simple, long, spine-like, and laterally com- 

 pressed. Stylets with second segments (double?) 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 Crustacea, are 

 so rarely preserved in fossil species, especially those of smaller sizes, renders 

 their study more difficult than that of most organic remains. Hence, in descri- 

 bing new species, genera, or other groups, the palaeontologist 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 investigation 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, that we meet with 

 types presenting a combination of characters which in existing species are dis- 

 tributed 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 prdbably the next specimen found would show it to possess some 

 one or more wholly incompatible 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 Aniphi- 

 pod. A more careful examination, however, soon showed that it presents 



