THE EURYPTERIDA OF NEW YORK 331 
and Pt. raniceps Salter; the marginal, compound eyes; the 
slender body with slight constriction between the anterior and posterior 
abdominal portions; the cordate form of the metastoma; and the simplicity 
of the opercular plate and its appendage. The two forms of this appendage 
resemble those of Pterygotus as far as the material of the latter permits 
of comparison, one form being a slender, sagittate-based scale, the other 
a shorter, broader, convex body, as in Pt. bilobus Salter and 
Pt. osborni Hall. In the new genus the first form projects 
posteriorly to protect a shorter lance-linear appendage lying in a cleft 
of the second sternite, while the second form is followed by an entire ster- 
nite without appendage. The sixth pair of cephalothoracic appendages 
resemble those of Pterygotus in being only slightly expanded distally and 
consisting of eight joints, with a rudimentary ninth inserted in the end 
of the palette. The gnathobases have the upright retort form. 
The outline of the carapace is very much alike in all species, recall- 
ing Pterygotus banksii Salter.1. The latter, however, is by 
no means a typical Pterygotus, but in various characters, as notably also 
the form of the carapace, is quite apart from the other Pterygoti. On the 
other hand, the carapace of H. socialis resembles most closely 
that of a species described by Salter and Woodward as an Eurypterus 
(E. lanceolatus).’? The latter is, in our opinion, either a true 
Hughmilleria or a transitional form between Hughmilleria and Eurypterus; 
at any rate it points to a close relationship of the two genera. 
As the second character suggestive of Pterygotus, ‘‘the marginal 
compound eyes’”’ are cited. We see in the structure of these eyes in 
Hughmilleria strong evidence of the intermediate position of the genus 
between both Eurypterus and Pterygotus. The facts are the following: It 
is well known that Pterygotus possesses distinctly faceted compound 
eyes of elliptic to circular outline, the whole ocular node being occupied 
by the visual area. In Eurypterus, on the other hand, the facets have 
‘Pterygotus raniceps, also adduced for comparison by Sarle, has a more 
acutely triangular carapace and is here referred to Eusarcus. 
*In discussing the resemblances between Eurypterus and Hughmilleria [cbzd. 
p- 10go], Sarle correctly indicated the similarity of Hughmilleria to Eurypterus 
lanceolatus inthe form of the carapace and the swimming arm. 
