Aug., i8 9 3. EURYPTERIDA. 125 



feet." This would make the whole number of appendages six instead 

 of five, as described by Huxley, who only recognised three pairs of 

 walking legs. 



In Slimonia and Enryptevus the full complement of post-oral 

 appendages (five pairs) has long been recognised. In the former the 

 first pair, described by Woodward (3), and by him termed "antennae," 

 is modified to form tactile organs, but is undoubtedly post-oral in 

 position, as the basal joint bears teeth. Pre-oral appendages have 

 been described in Eurypterus fischeri by Schmidt (4) as a pair of fine 

 antenniform structures. In Slimonia and in Eurypterus scorpioides and 

 E. conicus the pre-oral appendages have been described (6) as small 

 stout pincers, much like the chelicerae in Limulus, and corresponding 

 generally to the big pincers in Pterygotns, though probably masticatory 

 in function rather than prehensile. The difference between these 

 appendages as described in E. fischeri and E. scorpioides, etc., is very 

 great, but we must wait for further evidence before making the 

 apparently logical change of dividing Eurypterus into two distinct 

 genera, or even families. There would be a certain awkwardness in 

 founding one's classification on a structure which has only been 

 described in three species out of the forty or fifty in the genus. 



The legs in the other genera of Eurypterida are less well-known. 

 In Stylonurus the last two pairs are enormously elongated. The other 

 legs are only known by fragments, but among the fragments figured 

 in a recent paper by Hall and Clark (5) is a well-marked pair of 

 chelicerse, which were no doubt pre-oral in position. Limbs inter- 

 mediate in form between the elongated type of Stylonurus and the 

 broad " swimming feet " of Eurypterus have been described in 

 Dolichopterus (2) and Drepanopterus (8). 



The hard parts, other than appendages, on the ventral surface of 

 the carapace consisted of the epistoma in front of the mouth and the 

 metastoma behind it. The epistoma was described by Huxley and 

 Salter in Pterygotns, but they were misled by the direction of the scale 

 markings on it, and tried — as pointed out by Schmidt — to get it in 

 hind side before. It consists in Pterygotns, in which it is best known, 

 of a thin, semicircular plate, which covered the space between the 

 anterior margin of the carapace and the mouth, the pre-oral appendages 

 being attached close to its posterior border. The reversed position of 

 the scale markings, which have their convex side directed forward, 

 would seem to point to its being morphologically the inturned front 

 border of the carapace. It probably corresponds to the hypostoma of 

 Trilobites, which occupies the same position. 



The metastoma is a heart-shaped plate attached along the middle 

 line to the ventral wall of the body, between the bases of the last pair 

 of legs and extending outwards and forwards so as to enclose the 

 jaws in a kind of chamber. 



The margins of the carapace are often bent over on to the ventral 

 surface to a greater or less extent. This is particularly well-marked 



