34 NEW YORK STATE MUSEUM 
This glabellar ridge with its bounding furrows is most dis 
tinctly 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 (ec. gz 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, inE. lacustris andremipes, 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. ro] 
it becomes very broad and concentrically striated. In Limulus the 
doublure broadens in the median part of the anterior portion into a con- 
cave triangular shield that is said to serve as an inlet to the water for res- 
piration 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 lke function. 
To the doublure are attached by an open suture two plates, figured 
by Hall [18s9. 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 
