ON THE SYNCARIDA. 127 



or less acnte at the end. There is nothing in the strnctnro of the urosome and its uropoda in 

 Acanthotelson to remind us of the same parts in the Ampliii)oda. 



Excluded from the sessile-eyed Crustacea, and fonied to place Acanthotelson in the Thoracos- 

 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 api)endages and telson is 

 such as to forbid us from rejecting the Syncarida 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 Syncarida, from their lack of a carapace, and from the 

 well-formed dorsal arch of the seven thoracic segments, we are obliged to consider as an annectant 

 or synthetic group, pointing to the existence of some extinct group which may have still more 

 closely connected the sessile-eyed and stalked-eyed Crustacea. 



Notice of Acanthotelson ? magister (n. sp.). 



PL II, 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- 

 nse are not preserved sufiflciently 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. II, tigs. 

 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 antennse crossed the end of the abdomen with its a[)pend- 

 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 anteuni* ; one fragment, 

 the innermost, showing traces of six joints ; and there are faint impressions, not showing the joints, 

 of two long antenniB, 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 vei'y broad, being nearly twice as broad as the largest Acanthotelson eveni, 

 M. & W. The penultimate abdominal segment is 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 setae. The basal joint is broad, triangular, but a 

 little longer than broad ; the outer ramus is of moderate length, eusiform, 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 antennae or telson, TO""™. . 



Length of pennltimate abdominal segment, 5™™. 



Breadth of jjenultimate 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. it W., restored, enlarged twice. 



Fig. la. Acantlwlelson slimpsoni M. & W., head and antennae seen from above, enlarged twice. 



Fig. lb. Acanthotelson stimpsoni M. & W., first thoracic leg x\. 



Fig. Ic. Acanthotelson stimpsoni M. & W., sixth thoracic leg xf. 



Fig. Id. Acanthotelson stimpsoni M. & W., telson and last pair of uropoda X^. 



Fig. 2. Acanthotelson f magister Pack., X\. AH the figures drawn by Dr. J. S. Kingsley. 



