NATURAL SCIENCES OF PHILADELPHIA. 47 



Dana, that however important this character may be in the study of the re- 

 cent Tetradecapod Crustacea, it can scarcely be made available in the inves- 

 tigation of crushed fossil species, where so many accidents might have oc- 

 curred to place the legs in an unnatural position. 



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

 tacean 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 tho- 

 racic 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 indi- 

 cate that the lateral motion of the thoracic legs was in some way restrained 

 so as to prevent the animal from taking an erect position, which is precisely 

 the effect produced in the normal Amphipoda by the possession of well de- 

 veloped squamose epimerals. If this should prove to be the case, it would 

 show that the remarkable combination of Amphipod and Isopod, or Anisopod 

 characters, already alluded to in this fossil, are real, and not simulative ; 

 since it would thus present mainly the anterior structure (possibly even to 

 the thoracic position of the branchiae) of a normal Amphipod, combined with 

 the single pair of styliform, and five natatory abdominal appendages of the 

 Itsopoda or Anisopoda. 



It must be evident, we think, that such an ensemble of characters as that 

 presented by our fossil, would exclude it from any known family of the Te- 

 tradecapoda ; hence we can but regard it as the type of a new family. 



ACANTHOTELSON SlIMPSONII, M. & W. 



Linear or sublinear in form. Upper antennas at least as long as the head 

 and first five thoracic segments ; peduncle moderately stout, rather longer than 

 the head ; first joint a little longer and wider than the two others, which are 

 of nearly equal length ; flagellum slender and very minutely jointed ; acces- 

 sory appendage nearly or quite as long as the flagellum, and like it, 

 minutely jointed. Inferior antennae as long as the head and seven thoracic 

 segments; peduncle slightly longer, but otherwise similar to that of the 

 upper antenna? ; flagellum a Utile stouter and longer, but in other respects as in 

 the upper pair. Head, as seen in the (compressed) side view, subquadrangular, 

 longer on the upper than the lower side, in consequence of the obliquity of 

 anterior side ; eyes small, round, placed just below the bases of the upper 

 antennae. The (fourteen) thoracic and abdominal segments distinct, and 

 (excepting the last one) of nearly equal length, a few of those nearest the 

 head being a little shorter than the others ; all diminishing in depth (side 

 view) from about the antepenultimate one forward ; their anterior baBal 

 margins rounded ; posterior rectangular, or a little rounded. 



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 other joints above. Five succeed- 

 ing pairs of legs of nearly equal size and form ; their upper two (or three?) 

 joints 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 joint short and quadrangular; 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 seta?. Telson as long as the last four abdomi- 

 nal segments ; at its base one-half as wide, vertically, as the penultimate 



1865.] 



