ON THE SYNCARIDA. 127 
or less acute at the end. There is nothing in the structure of the urosome and its uropoda in 
Acanthotelson to remind us of the same parts in the Amphipoda. 
Excluded from the sessile-eyed Crustacea, and foreed to place Acanthotelson in the Thoraéos- 
traca, we are confronted by the lack of a carapace and the homonomous segmentation of the body. 
These are essential fundamental characters, but still the nature of the appendages and telson is 
such as to forbid us from rejecting the Synearida from the ordinal limits of the Thoracostraca. 
We are compelled, therefore, to regard the group as a suborder standing near or at the base of the 
Thoracostraca, not far from the Stomapoda and Schizopoda, and with appendages closely homol- 
ogous with those of these two groups. The Synearida, from their lack of a carapace, and from the 
well-formed dorsal arch of the seven thoracie segments, we are obliged to consider as an annectant 
or synthetic group, pointing to the existence of some extinet group which may have still more 
closely connected the sessile-eyed and stalked-eyed Crustacea. 
NOviIcE OF ACANTHOTELSON ? MAGISTER (n. sp.). 
Pl. Il, Figs. 4,5. 
I have received from Mr. J. C. Carr, for examination, a specimen from Mazon Creek, collected 
at the same place as the nodules containing the Acanthotelson, showing the remains of a crustacean 
closely similar to, if not generically identical with Acanthotelson. Unfortunately the head and anten- 
ne are not preserved sufficiently well for description, so that the following account should be regarded 
as provisional, until better-preserved specimens are found. As seen by the photograph (PI. I, figs. 
4,5), the animal was of the same general shape as in Acanthotelson; when it died the body was 
curved on itself, so that the two longer antenne crossed the end of the abdomen with its append- 
ages. The abdomen in its dorsal aspect, with the telson and last pair of uropoda, are tolerably 
well preserved. The faint traces of the head, unless we are mistaken, show that it was of the 
same general shape as in Acanthotelson, There are traces of two pairs of antenniwe; one fragment, 
the innermost, showing traces of six joints; and there are faint impressions, not showing the joints, 
of two long autenne, which are about half as long as the body. There are no traces of any thoracic 
or abdominal appendages except the last pair of uropoda. 
Description.—Body very broad, being nearly twice as broad as the largest Acanthotelson eveni, 
M.& W. The penultimate abdominal segmentis a little more than one-half as long as the terminal 
segment. The last segment is very large and square, the sides nearly even, not narrowing poste- 
riorly, and it is the broad square shape of this segment which will readily enable one to separate 
it from the previously described species of Acanthotelson. The telson is stout, broad at the base, 
and rather short, much shorter than the uropoda appended to the same segment. The terminal 
uropoda are broad and stout, with no traces of set. The basal joint is broad, triangular, but a 
little longer than broad; the outer ramus is of moderate length, ensiform, and slightly longer than 
the telson; there is only a fragment of the inner telson left in the fossil, which, however, shows 
that it was considerably narrower and smaller than the outer pair. 
Probable length of the whole body, not including the antenni or telson, 70", 
Length of penultimate abdominal segment, 5™™, 
Breadth of penultimate abdominal segment, 12™™, 
Length of terminal abdominal segment, 10™™, 
Breadth of terminal abdominal segment, 11™™, 
Length of telson, 10™™; breadth at base, 2™™, 
Length of basal joint of last pair of uropoda,4™™; breadth, 3.5™™, 
Length of outer ramus of last uropod, 11™™; breadth, 2", 
Explanation of Plate I. 
Fig. 1. Acanthotelson stimpsoni M. & W., restored, enlarged twice. 
Fig. la. Acanthotelson stimpsoni M. & W., head and antennx seen from above, enlarged twice. 
Fig. 1b. Acanthotelson stimpsoni M. & W., first thoracic leg x %. ‘ 
Fig. le. Acanthotelson stimpsoni M. & W., sixth thoracic leg x#. 
Fig. 1d. Acanthotelson stimpsoni M. & W., telson and last pair of uropoda x #. 
Fig. 2. Acanthotelson ? magister Pack., x}. All the figures drawn by Dr. J. S. Kingsley, 
