THE EURYPTERIDA OF NEW YORK 35 
of the carapace from further sinking in and therefore appear on the com- 
pressed carapace as a very marked smooth, flat marginal zone [pl. 6, 
fig. 5]. In Pterygotus a third plate is intercalated between the two 
marginal plates in front of the mouth, forming an epzstoma that occupies 
the same position as the hypostoma of the trilobites. 
Limulus possesses in front of the chelicerae a wartlike node on the 
ventral membrane which has been shown by Patten [1894] to contain an 
olfactory organ. While Holm’s photographs demonstrate the absence of 
anything similar in Eurypterus fischeri, it seems to us that 
nodes observed in specimens of Eusarcus and Stylonurus in the cor- 
responding place may possibly indicate the presence of a hke organ in 
these genera. | | 
The connection of the carapace with the abdomen is accomplished by 
articulations near the postlateral angles, well seen in plate 6, figure 
5. It is indicated by an abrupt change in the direction of the posterior 
margin where the truncation of the genal angle begins. Between the 
articulations the margin of the carapace is curved forward, so that an 
open slitlike space remains between the carapace and the abdomen which 
is occupied by a thin membrane connecting the doublures of the carapace 
and first abdominal segment. As Holm has pointed out, the open space 
- indicates that the movability of the articulation between the carapace and 
the abdomen must have been very considerable. 
_ Eyes. The carapace bears two pairs of eyes, the large lateral or 
compound eyes and the median eyes or ocelli. The lateral eyes distinctly 
fall into two groups by virtue of structure and position. 
The first of these groups exhibits a smooth visual area which is more or 
less crescent-shaped and borne on an elevated ocular node between the glabella 
and the lateral margins. This type of eye is exemplified by Eurypterus 
and is also found.in Dolichopterus, Drepanopterus and Stylonurus. Prob- 
ably Strabops also possessed eyes of this type; Eusarcus, in regard to its 
eyes as well as its whole body form, 1s an aberrant type, for while it possesses 
apparently smooth bean-shaped eyes these are marginal as in those of 
