As minor characters which, however, also contribute to give Eusarcus its peculiar or even odd appearance and which seem to be present in all the species before cited, may be mentioned the strong development of the spines of the walking legs, the relatively great length of the seventh segment of the swimming legs and the peculiar surface sculpture, which in all representatives shows a strong tendency to become tuberculate instead of triangular-scaly as in the others, i.e. the scales are much smaller, more crowded and more or less circular in outline.
Three of the generic characters are closely correlated and combined. They indicate an attitude of the animal wholly different from that of Eurypterus. These are the triangular carapace, the frontal position of the compound eyes and the predominant size of the anterior walking legs. These characters demonstrate that Eusarcus raised the eyebearing front end of its carapace highest above the ground while Eurypterus brought its broad shovel-shaped frontal part down to the ground in walking.
Another peculiarity is intimately connected with this style of bearing the carapace. Though at first glance it is apparently of minor importance, it gives species of Eusarcus a very different aspect from those of Eurypterus and indicates a difference of habit in the two genera. In Eurypterus the walking legs are invariably bent backward in the fossil state, while in Eusarcus they are as invariably bent forward.[1] In the living animals their direction was downward in Eurypterus as well as in Eusarcus but while in the former the posterior spines are the longer, in Eusarcus the anterior ones were either longer or at least of equal length with the posterior. The greater length of the posterior row of spines in Eurypterus must have emphasized the effect of the greater length of the posterior legs in bringing the front of the carapace down and, likewise, the greater length of the anterior row in Eusarcus would have increased the effect of the greater length of the anterior legs and assisted in lifting up the front
- ↑ With the exception of the small Eusarcus obesus (probably a young form) while in the British Eusarcus scorpioides they are as strongly bent forward as in our two species.
