1866.] WOODWAED STETJCIUEE OF XIPHOSrEA. 31 



Fterygotus, as in the recent Limvhis, the reproductive organs, and 

 succeeded, no doubt, in both, by similarly formed membranous ap- 

 pendages beariug branchiae. 



This latter point, I think, is established on the evidence of speci- 

 mens both in the Museum of Practical Geology and in the British 

 Museum, showing two opercular-shaped plates associated together 

 (PL II. fig. 11) and evidently belonging to the same individual. 

 One plate also exhibits tvro small rounded prominences, which, 

 there can be little doubt, were ovarian openings. 



If the head in LimuJus be composed of the cephalothorax, and 

 the posterior portion be the abdomen, then it follows that the oper- 

 cular plate is thoracic, and the succeeding branchial plates are abdo- 

 minal, thus differing widely from the fossil forms, in which the 

 head is simply the head, with one thoracic segment added to it 

 bearing the opercular plate; the branchiae are thoracic, and the 

 abdomen is entirely destitute of appendages. 



Let it be granted, however, that the head in Limulus represents 

 the entire cephalothorax and the posterior portion the abdomen ; 

 if specimens can be shown having a " postabdominal " series of 

 segments, I think it will be conceded by all carcinologists that in 

 Limulus the abdomen is rudimentary, as is the case in Cyclops, 

 Daplinia, Lerncea, &c., and in the Brachyurous Decapods. 



In 1864 I exhibited at the Bath Meeting of the British Associa- 

 tion a form which I named Hemias^is (PI. I. fig. 3 ; and Quart. 

 Journ. Geol. Soc. 1865, vol. xxi. p. 490, pi. xiv. fig. 7, a, 6)*. We 

 have in Hemiaspis a limuloid crustacean, having a head-shield, six 

 free and moveable segments forming the thorax, and three very nar- 

 row, and apparently double, segments representing the abdomen, 

 followed by a long and slender " telsonj' 



Since describing the above remarkable form, I have lately pro- 

 cured a paper, published at Dorpat, by Dr. J. Meszkowskif, by 

 which I have become acquainted with two new forms from the 

 Upper Silurian of the Island of (Esel, which must be added to the 

 list of forms allied to Hemiaspis. They are named by Dr. Mesz- 

 kowski P5^MC?oni5Ci(S aculeatiis (PL T. fig. 5) and Exap)inurus Schrenhi 

 (PL I. fig. 6). 



Professor Eichwald has described another, which, no doubt, is 

 related to this group of Crustacea, namely Bunodes lunula (PL I. 

 fig. 4). All these forms have tliree well-marked di\'isions to their 

 bodies, of head, thorax, and abdomen ; and all, save Bunodes, posses- 

 sed a telson, or tail-spine, and free articulated thoracic somites. 



Having been favoured with the loan of the very beautiful series 

 of Coal-measure Limidi belonging to our Treasurer, J. Prestwich, 

 Esq., F.E.S., and also with a series from the Broseley Institute, and 



* I avail myself of this opportunity to correct an error in the description of 

 Heoniasjpis liravJoides. The portion figured as h on the plate referred to, and 

 described as the centre of the glabella, is in reality that of another species, the 

 glabella in this species being nearly smooth, as in the figure given herewith, 

 drawn from a specimen in the late Islrc. H. Wyatt-Edgell's collection. 



t Op. ct loc. cit. pp. 380, 881 ; and p. 384, figs. 12-15. 



