1/4 ANNULOSA. 



species in the Devonian, and extends into the Carboniferous 

 Rocks. Hemiaspis, with only nine segments and the telson 

 behind the carapace, is exclusively Upper Silurian. Lastly, 

 Pseudoniscus, with the same number of free segments, is found 

 in the passage-beds between the Upper Silurian and Devonian. 

 In conclusion, it is interesting to note that these ancient 

 Crustaceans present many larval features, resembling the larva? 

 of the Decapoda, especially in the fact that the hinder portion 

 of the body is composed of free segments, which carry no 

 appendages.* 



SUB-ORDER II. XIPHOSURA (Pcedlopoda). 



" Crustacea having the anterior segments welded together to 

 form a broad convex buckler, upon the dorsal surface of which are 

 placed the compound eyes and ocelli ; the former sub-centrally, the 

 latter in the centre in front. The mouth is furnished with a small 

 labrum, a rudimentary metastoma, and six pairs of appendages. 

 Posterior segments of the body more or less free, and bearing upon 

 their ventral surfaces a series of broad lamellar appendages ; the 

 telson, or terminal segment, ensiform." (Henry Woodward.) 



The only living members of the Xiphosura are the Limuli, 

 commonly known as King-crabs or Horse-shoe Crabs. The 

 anterior portion of the body is covered by a broad horse-shoe- 

 shaped buckler (fig. 1 1 7), the upper surface of which bears a pair 

 of larval and a pair of compound eyes. On the lower surface 

 of the carapace is placed the aperture of the mouth, surrounded 

 by six pairs of limbs, the bases of which are spinous, and 

 officiate as jaws, whilst their terminations are converted into 

 chelae or nipping-claws. The first pair of appendages is placed 

 in front of the mouth, and represents the antennae, so that the 

 antennae of the King-crabs are chelate. Behind the cephalic 

 buckler comes a second shield, composed of six amalgamated 

 segments, below which are carried the reproductive organs and 

 branchiae, the former protected by a thoracic plate or " oper- 

 culum," the latter borne by five pairs of lamellar appendages. 

 Lastly, articulated to the posterior margin of the abdominal 

 shield, is a long sword-like spine or "telson" (fig. 117, t). 



The Xiphosura seem to have commenced existence in the 

 Upper Silurian period, where they are represented by the 

 Neolimulus falcatus of Mr Henry Woodward. With this ex- 



* The student desirous of fuller information on this subject, as on the 

 Xiphosura also, should consult the excellent memoirs by Mr Henry 

 Woodward, and especially his monograph of the Merostomata, published 

 by the Pabeontographical Society. 



