Page:The Eurypterida of New York Volume 1.pdf/63

 the median line and having a median lobe attached to them. The plates have a straight anterior margin and frequently well rounded antelateral angles. The scales form a continuous transverse line across the plates which has been erroneously considered as the suture resulting from the fusion of the two sternites. In front of the median lobe two triangular—or sometimes pentagonal—areas are marked off [ pl. 11, fig. 3] by sutures from the opercular plates in Eurypterus and Slimonia but rarely in Pterygotus. Laurie suggests that these areas may represent the paired sternite of the first abdominal segment, the remaining portions of the plates representing the appendages.

The middle lobe shows two different forms not known to Hall but which were recorded by Woodward in  and   [1863, p. 61; 1872, p. 114, f.] and attributed to sexual differences. Schmidt likewise recognized two forms of opercular appendages of sexual significance in, and Holm, by reference to Limulus, agreed with Woodward in assigning the more primitive appendage to the male and the more elaborate to the female, thus bringing out the fact that the mature males, at least in Eurypterus, are smaller than the females, as is true of Limulus. Gaskell, however, asserts [1908, p. 191] that the operculum of the eurypterids belonged to the type of Thelyphonus rather than to that of Limulus or Scorpio and as appears from his diagram [see text fig. 18] he would, on the strength of this claim, reverse the reference of the appendages to the sexes. While it may be that the elaborate opercular appendages of the eurypterids exhibit less similarity to the extremely primitive exterior genital apparatus