from the English Coal-measures. 159 



and with many of the Cumacea, we must suppose this coalescence to 

 be a very ancient character of the Older. Another important feature 

 is the abbreviation of the abdomen, which is shorter than the thoracic 

 region in all Isopoda and Tanaidacea. If we may assume, from the 

 evidence of the Tanaidacea, that the acquisition of these characters 

 preceded the suppression of the carapace in the evolution of the 

 Isopoda, then the free telson and elongated abdomen of Pleurocaris 

 suggest that it belongs to another line of descent. 



The only Crustacea with which, so far as I know, Pleurocaris can 

 be closely compared, are the species of Acanthotelson from the Coal- 

 measures of Illinois. 1 The similarity is especially close as regards 

 the tail-fan, both genera having the telson long and tapering to a sharp 

 point, and the rami of the uropods very slender and armed with 

 marginal spines. Acanthotelson, however, has no thoracic pleural 

 plates, and has the first of the seven thoracic legs stonter than the 

 others, and armed with large spines. The somites also lack the 

 transverse ridges so conspicuous in the species here described. These 

 differences seem to justify the establishment of a new genus for the 

 English species, but it can hardly be doubted that the two genera are 

 very closely allied. 



I have previously expressed the view 2 that Acanthotelson is closely 

 related to Uronectes (Gampsonyx), JPrceanaspides, and certain other fossil 

 genera, which, with the living Anaspides, Paranaspides, and K oonung a, 

 I have grouped together under the name Syncarida. To this view 

 I still adhere, although I now agree with Mr. Stebbing 3 in thinking 

 that less reliance is to be placed on Packard's restoration of Acantho- 

 telson than on the original figures of Meek & Worthen. I cannot, 

 however, agree with Mr. Stebbing that Acanthotelson has any but the 

 most superficial resemblance to Apseudes or any other Tanaidacea. 

 The possession of a distinct carapace coalesced with the first two 

 thoracic somites is a universal and no doubt primitive character of that 

 Order, while in Acanthotelson the second somite is certainly distinct 

 and possibly the first also. Mr. Geoffrey Smith has suggested 4 that 

 Acanthotelson may be a "generalized Amphipod", but 1 am unable 

 to find any support for this view in the known characters of the 

 fossil. Acanthotelson appeals to agree with Uronectes and with 

 Prceanaspides in the segmentation of the thorax, and it resembles the 



1 The genus Acanthotelson was established by Meek & Worthen in 1865 

 (Proc. Acad. Nat. Sci. Philadelphia, 1865, p. 46) for two species, A. stimpsoni 

 and A. incequalis, which were more fully described and figured in the following 

 year (Geol. Surv. Illinois, ii, p. 399). In 1868 (Amer. Jour. Sci. (2), xlvi, 

 p. 27, and Geol. Surv. Illinois, iii, p. 549) the same authors added a new 

 species, A. eveni, and withdrew A. incequalis as founded on an imperfect 

 specimen of Palceocaris, giving also a fuller description of A. stimpsoni and 

 additional figures. Packard in 1886 (Mem. Nat. Acad. Sci. Washington, iii 

 (2), mem. 15, p. 123) discussed the genus, for which he established the group 

 Syncarida, and gave a restoration of A. stimpsoni. 



2 Trans. Boy. Soc. Edinburgh, xxxviii, pi. iv, p. 799, 1896 ; Ann. Mag. Nat. 

 Hist. (7), xiii, p. 155, 1904 ; Lankester's Treatise on Zoology, pt. vii, fasc. 3, 

 Crustacea, p. 167, 1909. 



3 Natural Science, xi, p. 252, 1897. 



4 Quart. Jour. Micr. Sci., liii, p. 500, 1909. 



