ANATOMY AND RELATIONS OF THE EURYPTERID^E. 519 



Such a change in classification, however, based upon structures so seldom preserved, 

 would, even if logically correct, be practically a great disadvantage, as it would be 

 impossible to say to which section any Eurypteri, other than the above three, should 

 be relegated. Altogether it seems more advisable to wait until one is compelled by 

 a large mass of evidence before making a change which would certainly be trouble- 

 some, and may prove to be unnecessary. 



The ventral surface of the abdomen is described by Schmidt as being covered by five 



pairs of plate-like appendages, each pair being united in the middle line. He further 



states that there are no ventral hard parts except these plates, and the ends of the dorsal 



sclerites, which are bent round on to the ventral surface. I have unfortunately not been 



able to confirm his observations, and am inclined to doubt the absence of ventral sclerites, 



is many comparatively well preserved specimens show no sign of a line down the centre 



>f the ventral plates. I do not, however, feel at all confident of the correctness of my 



aterpretation of these structures, as it is evident from his figures that Schmidt had very 



mch better material on which to make his observations than has fallen to my lot. 



Stylonurus. 



I have been unable to make out any new details of the structure of this form. The 

 form of the body is simple, and more like that of Pterygotus than Eurypterus, though 

 the dorsal position of the eyes ally it to the latter. The in turned portion on the under 

 side of the carapace is remarkably broad, and, owing to the chronic absence of the limbs, 

 is not unfrequently well shown. The presence of well-marked epimera on the posterior 

 segments in some forms, reminds one of some of the Hemiaspidse, but the resemblance is 

 only a superficial one. The form of the two last pairs of legs, which are long and pointed 

 at the end, and are among the most characteristic structures of the genus, is possibly 

 derived from Eurypterus through some form like Drepanopterus, though it is also 

 possible that Stylonurus is descended from an ancestral type in which the last pair of 

 legs were less modified than in Eurypterus. Of the other appendages comparatively 

 little is known. Woodward in his restoration of this form* figures five pairs of 

 appendages, the most anterior of which are antenniform. This pair has not, I believe, been 

 seen, and whether it was distinctly modified for a tactile function, as in Slimonia, or more 

 closely resembled the other walking limbs, as in Eurypterus, is a matter for conjecture. 

 A chelate appendage has recently been figured by Hall and Clarke t in St. Excelsior, 

 which they describe as follows (p. 222) : — " Directly behind the base of the right 

 member of this pair lies a single joint terminating in a chela, the whole measuring 

 60 mm. in length. The other joints of this appendage do not appear on this specimen, 

 and it is impossible to determine positively whether this is, as it seems, the terminal 

 portion of a third gnathopod or is analogous to the chelate antennules of Limulus." 



* hoc. cit., p. 131. t Hall and Clarke, Geol. Surv. New York, Palaeontology, vol. vii. 



