552 CRUSTACEA. 



sculpturing. The anterior part of the body is covered with a cara- 

 pace, which carries a pair of large, marginal, or subcentral eyes, and 

 a pair of small ocelli placed near the centre. On the under side of 

 the carapace is the opening of the mouth, furnished behind with a large 

 undivided metastoma and with a series of jointed appendages {five or 

 six pairs in number), of which the first are prceo?-al, and represent 

 antennce, zvhile the remainder are modified for mastication or locomo- 

 tion, the last pair fo7'ming usually great sivimming-feet. Behind the 

 carapace the body consists of numerous free thoracico-abdominal seg- 

 ments, certain of the anterior of which are furnished below with 

 divided lamella, which concealed the branchitz. The hinder segments 

 carry no appe?idages, and the abdomeii terminates in a spine-like or 

 paddle-shaped telson. 



The integument of the Eurypterids is hardened by chitine, and 

 when transparent fragments are examined under the microscope, 

 these exhibit a reticulated structure, numerous clear rounded 

 corpuscles, of various sizes, appearing to be disseminated in a 

 horny matrix ; the crust thus assuming a punctated or porous aspect. 

 The characteristic " scale-marking " is the result of the development 

 at intervals of curved linear thickenings of the exoskeleton. 



The anterior part of the body in the Eurypterids is covered by a 

 comparatively short, semicircular, or subquadrate carapace, which is 

 considered by Woodward, Schmidt, and Zittel as representing the 

 head only, but which is regarded by Claus and Hall as a cephalo- 

 thorax. The upper surface of the carapace carries a pair of large, 

 prominent, faceted, compound eyes, which may be subcentral, or 

 may be placed on the margin of the head-shield. There is also a 

 pair of small ocelli, usually placed close together, subcentrally. 



On the under side of the carapace is the opening of the mouth, 

 surrounded by a series of jointed appendages. The first pair of 

 appendages (fig. 416, an) are prseoral in position, and represent the 

 antennae. They terminate in pincers in Pterygotus (fig. 416, an), 

 but are stated by Fr. Schmidt to have the form of very slender 

 jointed organs in Eurypterus. The remaining appendages are 

 similar to those of Limulus in having their bases spinous, and in 

 officiating as jaws. Woodward recognises four pairs of these ; but 

 according to Fr. Schmidt there are five pairs, the total number of 

 appendages on the under side of the head-shield being thus six 

 pairs. The anterior appendages behind the antennae may represent 

 mandibles and maxillae, and they have the form of slender jointed 

 organs, with dilated and serrated basal joints (fig. 416, gn). The 

 last pair of appendages, regarded by Woodward as maxillipedes 

 (fig. 416, mx), are of great size, and while their bases are dentated 

 to fit them for mastication, they are usually dilated distally so as to 

 form powerful swimming-paddles. The mouth is a longitudinal 



