NATURAL SCIENCES OF PHILADELPHIA. 49 



attitude ; as well as in the depressed, slender form of its abdomen and thorax, 

 U seems to present decidedly the aspect of a Macrural Decapod. Yet, on a 

 closer examination, we can see no traces of a carapace, the thorax being 

 apparently divided into seven segments, like those of the abdomen, and each 

 provided with a pair of legs, as in the Tetradecapoda. If we are not mistaken 

 in these latter characters, and we certainly believe we are not, it must show 

 a most extraordinary union of characters, which, amongst recent Crustacea, 

 belong to different primary divisions. From all that can be made out of its 

 structure, we are therefore inclined to view it as one of the " embryonic" or 

 " comprehensive" types, so often met with in various departments of palaeon- 

 tology, and which furnish the advocates of the Darwinian hypothesis with 

 some of their strongest arguments. 



For the present, this genus is placed, provisionally, along with the Tetra- 

 decapoda, though it cannot, we think, be included in any known family of 

 that division ; while if it should prove to be an embryonic or low type of the 

 Decapoda, it may be even necessary to establish for its reception, a division 

 of more than family importance. It is proper to remark here, however, 

 that we have not seen any one specimen showing the caudal appendages we 

 have described, along with the other characters of the thoracic and cephalic 

 members, mentioned above. One imperfect specimen shows the seven tho- 

 racic and five or six of the abdominal segments, with their legs and nata- 

 tory appendages, the head, antennae, and apparently their basal scales; 

 while another shows the caudal appendages, and all of the thoracic and 

 abdominal segments, very distinctly, without any of the other members. 

 The general agreement, however, of these specimens, in the parts preserved 

 in each, is such that scarcely a doubt can be entertained that they belong 

 to the same species. Yet, in order to prevent confusion, we would remark, 

 that in case they should prove to belong to different genera, or species, 

 that it is the form showing the head, antennae, thoracic and abdominal 

 segments, with their appendages, &c, that we regard as the type of the genus. 



Pal^iocaris typos, M. & W. 



Linear, with thorax slightly wider near the middle than the abdomen ; 

 thoracic and abdominal segments of nearly equal length. Inner antennae 

 equalling the length of the head and thorax ; peduncles stout, first joint a little 

 longer and wider than either of the other two, which are of nearly equal 

 length, aud minutely and closely setigerous on their inner margins ; flagel- 

 lum very slender, and minutely jointed ; accessory appendage nearly or quite 

 as long as the flagellum, and scarcely differing from it otherwise. Outer 

 antenna possibly a little longer than the others, peduncles slightly longer 

 than those of the other pair, and like them minutely setigerous in front ; 

 basal scales (?) oblong, about as long as first joint of peduncles, squarely trun- 

 cated. Thoracic legs slender and long, anterior ones apparently not longer 

 or larger than the others, none of them (so far as can be seen) chelate, or 

 with any of the segments enlarged ; all the others with the first two or three 

 joints very short ; fourth ? joint horizontally extended, tapering, and about as 

 long as four segments of the body; succeeding joints (in the specimen ex- 

 amined) very slender and abruptly bent downwards and backwards. Na- 

 tatory abdominal appendages acutely lancelinear, and some of them as long 

 as four of the abdominal segments. Telson nearly as broad at the base as 

 the penultimate segment, tapering, and as long as two and a half of the 

 abdominal segments ; minutely setigerous on each side. Stylets, with first 

 joint very minute; second with each division as long as the telson, and lan- 

 celinear in form, with pointed extremities, and parallel, more or less setiger- 

 ous margins. 



Length of head, thorax, and first six abdominal segments, 0-78 inch ; do. 

 of head, 012 inch; do. of the seven thoracic segments, 0-35 inch ; do. of 



1865.] 4 



