2G0 Mr. R. I. Pocock on the Taionoinj/ 



Part IV.— Classification of Kixo-Crahs. 



Order XIPHOSUILE. 



Family Xiphosuridae. 



Subfamily Xtphosusinje { = LimultTKT) , nov. 

 (PI. Y. fig. A and PI. VI. fig. 7.) 



Genital opercxilum with tlie distal or third segment of the 

 inner branch retained as a freely movable sclerite, distinct 

 both from tlie second segment, to which it is jointed, and 

 from the distal segments of the outer branch of this appendage, 

 •which is widely separated from its fellow of the opposite side 

 in the middle line. 



Entostemuim with two pairs of long antero-lateral processes. 



Second pair of prosoniatic appendages modified as clampers 

 in the male. 



Opisthofoma more vaidted, not so markedly hexagonal, 

 owing to the lesser prominence of the lateral angle, which 

 lies well in advance of the middle of the lateral border, 

 making the spiniferous edge much longer than the part of the 

 border that has no movable spines; the latter abruptly bent 

 downwards in the posterior two thirds of its length, the spike 

 that it bears lying in front of its middle and much nearer to 

 the ** waist " than to the spike preceding the first movable 

 Fpine ; posterior prolongation of opisthosoraa more prominent, 

 the inner edge straight and cutting the outer at an acute 

 angle. Lateral crest bordering the gill-chamber on the lower 

 Fide of the Oj)isthoson)a without a strong spike, but posteriorly 

 evenly continuous with a serrated ridge which borders the gill- 

 chamber behind. 



Lateral movable spines of opisthosoma alike in both sexes, 

 becoming })rogressively shorter from before backwards, and 

 gradually taj)cring from base to apex. 



Sixth proMvmtic appendage with its fifth segment thick, 

 subcylindrical, not longer than the sixth. 



Penultimate segment of endopodite of branchial appendages 

 of opisthosoma armed aj)ically on the inner side with a 

 movable spine. 



Distribution. Eastern coast of North and Central America. 



Genus XiPHOSURA, Gronovius {= Limulus, Miill., as 

 restricted by Fabricius). 



Apart from the features mentioned above under the headinf^- 

 Xiphosurina3, some of which should perhaps be considered 

 ratlier of generic value, this genua presents the following 

 characters : — 



