The Eurypterid which is best known is Eurypterus fischeri (Figs. [161], 162), which is found in the Upper Silurian rocks at Rootziküll in the Island of Oesel (Gulf of Riga). In the Eurypterids from other deposits the chitinous exoskeleton has been altered into a carbonaceous substance, but in the specimens from Oesel the chitin is perfectly preserved in its original condition; and since these specimens are found in a dolomitic rock which is soluble in acid, it has been possible to separate the fossil completely from the rock in which it is embedded, with the result that the structure can be studied more easily and more thoroughly than in the case of specimens from other localities. Consequently Eurypterus fischeri[[230]] may, with advantage, be taken as a type of the Eurypterida.
The general form of the body (Fig. [161]) is somewhat like that of a Scorpion, but is relatively broader and shorter. On the surface of many parts of the exoskeleton numerous scale-like markings are found (Figs. [162], 163).[[231]] The prosoma or cephalothorax consists of six fused segments covered by a quadrate carapace with its front angles rounded. This bears on its dorsal surface two pairs of eyes—large kidney-shaped lateral eyes and median ocelli (Fig. [161], b, a). The margin of the dorsal part of the carapace is bent underneath to form a rim which joins the ventral part of the carapace.
On the ventral surface of the prosoma (Fig. [162]) six pairs of appendages are seen, of which only the first pair (the chelicerae) are in front of the mouth. The chelicerae are small, and each consists of a basal joint and a chela, the latter being found parallel to the axis of the body; they closely resemble the chelicerae of Limulus. The remaining five pairs of appendages are found at the sides of the elongate mouth, and in all these the gnathobases of the coxae are provided with teeth at their inner margins and were able to function in mastication, whilst the distal part of each appendage served as an organ of locomotion. The posterior part of each coxa is plate-like and is covered (except in the case of the sixth appendage) by the coxa of the next appendage behind. A small process or “epicoxite” is found at the posterior end of the toothed part of the coxae of the second, third, fourth, and fifth pairs of appendages. The second appendage consists of seven joints, whilst the remaining four consist of eight joints; none of these appendages end in chelae. The second, third,[[232]] and fourth pairs of appendages are similar to one another in structure, but become successively larger from before backwards. These three pairs are directed radially outwards; each consists of short joints tapering to the end of the limb, and bearing spines at the sides and on the under surface, and also a spine at the end of the last joint.
Fig. [162].—Eurypterus fischeri, Eichw. Upper Silurian, Rootziküll, Oesel. Restoration of ventral surface; 1–6, appendages of prosoma; m, metastoma. Immediately posterior to the metastoma is the “median process” of the genital operculum. (After Holm.)
The fifth appendage is longer than the fourth and is directed backwards; its second and third joints are short and ring-like; the others (fourth to eighth) are long and similar to one another, each being of uniform width throughout; the last joint is produced into a spine on each side, and between these two is the movable end-spine; the other joints do not bear long spines as is the case in the three preceding pairs of appendages.
The sixth appendage is much larger and stronger than the others, and like the fifth, is without long spines. The coxa is large and quadrate; the second and third joints are short, like those of the fifth appendage; the fourth, fifth, and sixth joints are longer and more or less bell-shaped; the seventh and eighth joints are much larger than the others and are flattened.
The metastoma (Fig. [162], m) is an oval plate immediately behind the mouth; it covers the inner parts of the coxae of the sixth pair of appendages, and represents the chilaria of Limulus. But, unlike the latter, it is not a paired structure; nevertheless the presence of a longitudinal groove on its anterior part renders probable the view that it is derived from a paired organ.[[233]] The front margin of the metastoma is indented and toothed. On its inner side in front is a transverse plate, the endostoma, which is not seen from the exterior, since the front margin of the metastoma extends a little beyond it.
Behind the prosoma are twelve free segments, of which the first six form the mesosoma (Fig. [161], 7–12). The tergum on the dorsal surface of each segment is broad and short, the middle part being slightly convex and the lateral parts slightly concave; the external margin is bent under, thus forming a narrow rim on the ventral surface. The tergum of each segment overlaps the one next behind. The segments increase in breadth slightly up to the fourth segment, posterior to which they gradually become narrower.
On the ventral surface the segments of the mesosoma bear pairs of plate-like appendages, each of which overlaps the one behind like the tiles on a roof. On the posterior (or inner) surfaces of these appendages are found the lamellar branchiae, which are oval in outline (Fig. [165], d). Between the two appendages of the first pair is a median process which is genital in function; this pair are larger than the other appendages, and cover both first and second segments, the latter being without any appendages, and they represent the genital operculum of Limulus (Fig. [153], 10). The form of the operculum, more particularly of the median process, differs in the male and female. In that which is believed to be the female (Fig. [162]) the median process is long, and extends beyond the posterior margin of the operculum; it is formed of two small five-sided parts at the base which are united at the sides to the two plates of the operculum; behind this is a long, unpaired part, which is pointed in front; this, together with the remaining parts, is not joined to the side-plates of the operculum, so that the latter are here separated from one another. The third part of the median process is shorter than the second, and bears at its end a pair of small pointed and diverging plates, the tips of which reach to the middle of the third plate-like appendages. On the inner side of the operculum there are, in the female, a pair of curved, tubular organs, attached to the anterior end of the median process, where they open, the free ends being closed; the function of these organs is not known, but was probably sexual.