Page:The Eurypterida of New York Volume 1.pdf/37

 of the carapace from further sinking in and therefore appear on the compressed carapace as a very marked smooth, fiat 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 epistoma 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, it seems to us that nodes observed in specimens of Eusarcus and Stylonurus in the corresponding place may possibly indicate the presence of a like 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. Probably Strabops also possessed eyes of this type; Eusarcus, in regard to its eyes as well as its whole body form, is an aberrant type, for while it possesses apparently smooth bean-shaped eyes these are marginal as in those of