THE EURYPTERIDA OF NEW YORK 51 
segments of the legs. There is other evidence of their prehensile function. 
In Limulus the first pair of walking legs of the male becomes transformed 
at maturity into a hooklike grasping organ and in Eurypterus 
fischeri, the second pair of legs also develops in the mature male 
a hooked clasping organ. 
The first pair of postoral appendages probably also served in most 
genera as a tactile organ. This is very clearly indicated in Slimonia where 
it is directly developed into an antenniform appendage.’ Its small size 
in Eusarcus and Stylonurus which contrasts with that of the following legs, 
is also evidence that it could have aided little, if at all, in walking or 
swimming. 
The fifth pair of postoral appendages have been termed the ectognaths, 
ectognathites or swimming legs, because in most genera (Eurypterus, Dol1- 
chopterus, Eusarcus, Hughmilleria, Slimonia, Pterygotus) the terminal seg- 
ments are flattened into a paddle-shaped organ that is currently considered 
as having functioned in swimming. Hall figured the swimming legs of the 
crab Platyonichus ocellatus [1861, pl. 84A, fig. 6, 7] to point 
out the remarkable analogy in its structure with that of the last leg of 
Eurypterus; and Holm has carefully worked out the characters which so 
excellently adapted this organ for a swimming function [1899, p. 27]. The 
most important of these are the sharp, knifelike edge of the anterior mar- 
gins of the fourth to sixth segments, the thin blade of the seventh and 
eighth segments and their articulation, by which they were enabled to 
form a continuous oar blade at the time of the backward stroke, while 
in the forward stroke, the eighth segment could be turned backward 
on the seventh like the blade of a shears to diminish the resistance of the 
water. The oar blade form of the extended seventh and eighth segments is 
well shown on plate 4, figure 3; the reflexing of the eighth segment is 
1 The following pairs of walking legs also show a peculiar development in Slimonia 
in being equally as long as in Pterygotus, but having a fringe of long spines on the 
distal edges of the segments, evidently developed from the serrations seen in Eurypterus 
and other genera, while the paired spines are absent. 
