This glabellar ridge with its bounding furrows is most distinctly marked in the embryonic Limulus [text fig. 24] where it bears the ocelli at its anterior point. It there continues over the abdomen and gives this stage its well known trilobitic appearance. It is still more prominent in the fossil Limuli (e. g. L. walchi), than in the recent species and is therefore apparently an old character possibly inherited from the common ancestor of the merostomes.
The elevated area of the glabella manifestly served, as in Limulus, to receive the anterior portion of the heart, while the space between the carapace and the ventral membrane in front of the ocelli contained the liver. We find that the interior surface of the carapace sometimes exhibits in this region, in E. lacustris and remipes, anastomosing radiating lines similar to those seen in many trilobites (Conocephalites, Harpides, etc.), and which have been interpreted by Jaekel as liver impressions.
The edge of the carapace is bent under, forming a doublure. This is usually narrow as in Eurypterus and runs out toward the genal angles. In Stylonurus, however, and notably S. myops [pl. 52, fig. 10] it becomes very broad and concentrically striated. In Limulus the doublure broadens in the median part of the anterior portion into a concave triangular shield that is said to serve as an inlet to the water for respiration when the broad carapace is resting on the mud. A triangular area of like relation and relative dimensions is set off in some species of Stylonurus [pl. 46, fig. 11] and may have had a like function.
To the doublure are attached by an open suture two plates, figured by Hall [1859. v. 3, pl. 80A, fig. 12], meeting in the median line along a suture and together forming a horseshoe-shaped organ, which toward the mouth passes gradually and by an irregular contact into the very thin membrane that surrounds the coxal segments of the legs. One of these marginal plates is shown in place in plate 5, figure 6, where the carapace is partly removed. Frequently in specimens of Eurypterus remipes not completely flattened, they have prevented the marginal portion
