are, as a rule, furnished with serrations, which become more prominent in posterior direction.
The articulation of the telson is quite like that of Limulus, consisting of a broad transverse lower and a small upper segment and is mainly adapted to movement in a vertical plane.
II
MODE OF LIFE
The mode of life of the eurypterids has thus far been touched upon incidentally in the description of the "swimming feet"; the earlier writers have generally assumed that the eurypterids were active swimmers on account of the structure of these legs which Hall very properly compared with the quite similar swimming organs of the common "lady crab" (Platyonichus ocellatus) of our eastern Atlantic coast. Woodward pointed out that the position of the eyes of Pterygotus, half above and half below the margin, indicates that it can not have been a mud grubber and he has also said of P. anglicus that its large eyes, its powerful natatory appendages and the general form of its body suggest that it was a very active animal. Laurie [1893, p. 511; 1893, p. 124], on the other hand, has repeatedly advanced the view that the eurypterids were bottom crawlers and diggers in the mud. He says of the swimming leg of Slimonia [op. cit. p. 511]:
This appendage is always described as a swimming organ, but I am inclined to doubt the correctness of this interpretation of its function. The Eurypteridae appear to me, from their general build, more fitted for crawling than swimming, and I am inclined to explain this appendage as having been used by the animal to get a firm hold on the bottom, and probably also for digging out sand and covering itself, in much the same way that Portunus uses its very similar pair of appendages.
Holm concluded from his study of Eurypterus fischeri that the last pair of legs of that species was principally adapted to swimming. He says:
Die Hauptbewegung des achten Gliedes im Verhältniss zum siebenten scheint von vorn nach hinten gewesen zu sein, indem sich, wie schon von Fr. Schmidt hervorgehoben ist, das achte Glied bei der Bewegung über die dreieckige Platte wie ein Scheerenblatt über das andere schiebt. Die beiden Glieder müssen daher beim Schwimmen wie ein einziges Ruder-
