380 ARTHROPODA. 



in number, with grooved pleurae ; and both eyes and facial 

 sutures are totally wanting. The type-genus of the family 

 is Agnostus itself (fig. 237), which is represented by numer- 

 ous forms in the Upper Cambrian and Lower Silurian. 

 Microdiscus of the Upper Cambrian (sometimes placed in 

 the Trinucleidce) agrees with Agnostus in its want of facial 

 sutures and eyes, but it has four body-rings, and the axis of 

 the tail is segmented. Lastly, the Shumardia of the Lower 

 Silurian is like Agnostus, but has both the axis and lateral 

 lobes of the pygidium distinctly segmented. 



ORDER MEROSTOMATA. 



Crustaceans, often of large size, in which the mouth is fur- 

 nished with mandibles and maxillce, the terminations of which 

 become walking or swimming feet, or organs of prehension (figs. 

 238, 239). 



The order Merostomata comprises the two sub-orders of 

 the Xiphosuroi and Eurypterida. The former appears to 

 have commenced its existence in the Upper Silurian period, 

 and is represented at the present day by the Limuli or 

 King-crabs. The latter is wholly extinct, and is exclu- 

 sively Palaeozoic, none of its members being known out of 

 the Silurian, Devonian, and Carboniferous formations. 



SUB-ORDER I. EURYPTERIDA. 



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

 ments, the first and second (?) of which bear one or more 

 broad lamellar appendages upon their ventral surface, the re- 

 maining 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 

 metastoma, and five pairs of movable appendages, the pos- 

 terior of which form great swimming-feet ; the telson, or ter- 

 minal segment, extremely variable in form; the integument 

 characteristically sculptured" (Henry Woodward.) 



In the typical Eurypterids, such as Pterygotus (fig. 239) and 



