172 



ANNULOSA. 



of its members being known out of the Silurian, Devonian, and 

 Carboniferous formations. 



Fig. 117. Xiphosura. Limuluspoly- 

 fhcmns, viewed from below, c The 

 cephalic shield carrying the sessile eyes 

 upon its upper surface ; o " Operculum," 

 covering the reproductive organs ; b 

 Branchial plates , a First pair of anten- 

 nae (antennules) ending in chela. Below 

 these is the aperture of the mouth sur- 

 rounded by the spiny bases of the re- 

 maining five pairs of appendages, which 

 are regarded by Woodward as being 

 respectively, from before backwards, the 

 great antennae, the mandibles, the first 

 maxilla:, the second maxillae, and a pair 

 of maxillipedes. All have their extremi- 

 ties chelate. 



Fig. 118. Eurypterida. Pttry- 

 Fotiis A it Aliens, restored (after H. 

 Woodward), c c Chelate antenna : 

 o o Eyes, situated at the anterior mar- 

 gin of the carapace ; nt m The mandi- 

 bles, and the first and second maxilla: ; 

 M n The maxillipedes ; the basal mar- 

 gins of these are serrated, and are 

 drawn as if seen through the metas- 

 toma or post-oral plate, which serves 

 as a lower lip. Immediately behind 

 this is seen the operculum or thoracic 

 plate, which covers the two anterior 

 thoracic somites. Behind this are five 

 thoracic and five abdominal somites, 

 and lastly, there is the tclsou (t). 



SUB-ORDER I. EURYPTERIDA. 



" Crustacea with numerous, free, thoracico-abdominal segments, 

 the first and second (?) of which bear one or more broad lamellar 

 appendages upon their ventral surface, the remaining segments 

 being devoid of appendages ; anterior rings united into a carapace, 

 bearing a pair of larval eyes (ocelli) near the centre, and a pair of 

 large, marginal, or sub-central eyes ; the mouth furnished with a 

 broad post-oral plate, or mctastoma, and five pairs of movable 



