124 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 
which Acanthotelson belongs, also without doubt comprises the generic and specific characters. 
We will first give a description of the fossils themselves, based on the material we have had for 
examination, and then endeavor to point out those characters which we suppose to be the essential 
features of the group to which the genus belongs, and also to indicate the probable affinities to 
the other divisions or suborders of Malacostraca. It may be as well to say that, after examining 
some forty specimens, we are unable to distinguish between Acanthotelson stimpsoni and event, and 
are inclined to believe that the former is the young of the latter species. 
In Mr. Lacoe’s No. 501) the head is well preserved; the first arthromere or segment is considerably 
shorter than any of the succeeding ones; it is slightly less than two-thirds as long as the succeed- 
ing arthromere; it bears in front a well-marked, small, triangular rostrum, which is acute at the 
tip, and is about two-thirds as long as the segment itself; the edge of the rostrum is considerably 
raised, especially at the base. The front edge of the segment on each side of the rostrum is also 
margined with an elevated ridge. The surface of the segment is rather full and convex on each 
side, but not so decidedly so as the second segment. The second arthromere is about as long as 
those succeeding, though not quite so long as the sixth arthromere; on each side is a low boss-like 
swelling, situated obliquely, and prolonged in an oblique direction to the anterior outer edge. 
The second segment is distinctly separated by an impressed line from the first, but there is not a 
true articulation between them, so that the first and second cephalic segments may be said to be 
consolidated and to represent the carapace of the Schizopoda. The three succeeding segments 
have a transverse, uninterrupted, smooth ridge situated in the middle on the third, but in the fifth 
segment near the hind margin. The sixth and succeeding segments are smooth and even. The 
body is of even width to near the telson. The lower edges of the segments are evenly rounded, 
those of the hinder abdominal segments are more acutely rounded. 
We have been unable to detect any positive traces of the eyes, nor can we state whether they 
were sessile or stalked, though if they were present and sessile we do not see why they should not 
have been preserved in some of the specimens (particularly 501° and 406°)," 
The first pair of antennze seem to arise directly from each side of the small, short, rudimentary 
rostrum. The scape is three-jointed, and not very long and slender; second joint not so thick, 
and about one-fourth shorter than the first and twice as long as thick ; third joint long and slender, 
considerably longer than the second. The scape bears two flagella, which are long, slender, mul- 
tiarticulate branches of unequal length, of which the inner is the thicker and shorter, the outer 
flagellum much slenderer and longer, the entire length of the antennie being one-half that of the 
second or outer pair. The second pair of antennae have also a three-jointed scape (which is not 
accurately represented in Meek and Wortheu’s figure). The basal joint is short; second joint shorter 
than the first, with two unequal internal spines; third joint slightly longer than the second and 
much smaller; there are traces of a small antennal scale; the flagellum is long and slender, its 
entire length about half that of the body. 
There are twelve pairs of feet (506*,°), a pair to each segment situated between the head and 
penultimate uromere or abdominal segment; these, with the caudal pair of appendages, make in all 
thirteen pairs of legs. . 
The number of arthromeres or body-segments is sixteen, counting the head as consisting of two 
when seen-from above, and the telson as a rudimentary arthromere, so that there are thirteen 
arthromeres between the head and telson, each of them bearing legs. There is no apparent dis- 
tinction, as regards the segments themselves, into cephalothorax and abdomen (urosome), but there 
are two cephalic, nine thoracic segments, and seven abdominal, counting the telson as the seventh. 
The first seven pairs of (thoracic) legs are much alike in appearance, reminding us of those of Peta- 
lophthalinus and Gnathophausia; these are succeeded by five pairs of abdominal appendages, which 
are about half as long and large as the thoracic legs. The first pair of thoracic legs (which do not 
seem to be mandibular palpi) are considerably larger (broader and longer) than the succeeding 
ones. It is composed of six joints; the first and second rather narrow; the third broad, with, ac- 
cording to Meek and Worthen, “ three” spines on the “ under side” (these were not to be seen in 
my specimens, though undoubtedly existing there ); fourth longer than the third, with three spines; 
‘Before going to press I received from Mr, Lacoea very large specimen, his No. x4, in which are two large smooth 
concavities, one on each side of the base of the head; it is possible that these are sessile eyes. 
