THE EURYPTERIDA OF NEW YORK OI 
the median line and having a median lobe attached to them. The plates 
have a straight anterior margin and frequently well rounded ante- 
lateral 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 trian- 
gular—or sometimes pentagonal—areas are marked off [pl. 11, fg. 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 Pterygotus bilobus and 
Slimonia acuminata [1863, p. 61; 1872, p. 114, f.] and attributed 
to sexual differences. Schmidt likewise recognized two forms of oper- 
cular appendages of sexual significance in Eurypterus fischert, 
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 Euryp- 
terus, are smaller than the females, as is true of Limulus. Gaskell, how- 
ever, 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 
1In this connection Gaskell has introduced [op. cit. fig. 78, p. 191] a figure which 
is Stated to ‘“‘ be a picture from Schmidt of the ventral aspect of Eurypterus,”’ but in 
fact is a monstrous mixture of the characters of Eurypterus and Pterygotus, such as 
Friedr. Schmidt could not possibly have perpetrated even in a nightmare. This is a 
good example of the careless treatment of fossils in zoological textbooks and treatises, 
exemplified again by the figure ofa Pterygotus anglicus labeled as “ Euryp- 
terus remipes ’’ in the Texi-book of Invertebrate Morphology, by J. P. McMurrich. 
