John Mason Clarke

“ In Eurypterus and Eusarcus the transparent spots are distinctly thickened into lenslike bodies. The ocellar tubercle is always situated on the median line of the carapace and it wanders forward and backward with the lateral eyes so that as a rule it is on a cross line connecting the posterior extremities of the lateral eyes. Exceptions are made by the genera Eusarcus, Pterygotus and Slimonia where the lateral eyes have become marginal at the front of the carapace while the ocelli remain on the middle of the carapace, thus retaining the advantage of their position at the apex of the shield. ”

“ The postabdomen is the longest division of the body. It surpasses by one half the preabdomen, decreases gradually in width to nearly one half, while at the same time the length of the segments is doubled, in the first segment the width surpassing the length five and one half times, and in the last not even one half times (actual proportion 10 : 54; 20 : 28) . The postabdominal segments are furnished with distinct epimera that increase in width with each succeeding member, and are prolonged into short spurs, except on the last segment where they are produced into long, rounded lobes. ”

“ The postabdomen, always a slender part of the body with much lengthened posterior segments in later forms, is here short and broad, the segments not at all increasing in length backward. Altogether, all abdominal segments retain a highly primitive and simple aspect in their appearance as uniformly narrow straight bands that hardly bend forward at the lateral extremities. The first tergite, however, has progressed in development beyond the others, and, as in the Siluric genera, is more closely connected with the carapace than with the abdomen, and is distinctly bent forward at the extremities. ”

“ Huxley and Salter placed P. bilobus in a separate subgenus, Erettopterus, on account of its bilobed telson. While some have recognized this group as a distinct genus, others have not given it even subgeneric rank. Laurie states that the species with such telsons "might fairly be separated from the rest as a subgenus, were it not that the frequent absence of the tail would make such an arrangement highly inconvenient." In our view the recognition of a natural group can not be a matter of convenience, but is of necessity, when the differences are recognized as subgeneric in rank. ”

“ Hughmilleria is a good genus that requires distinction as being of a primitive or generalized character, through which it has points of similarity with a number of other genera. It is certain that, notwithstanding its exterior similarity to Eurypterus, it points, by virtue of its cordate metastoma, the intramarginal to marginal position of the compound eyes, the slightly longer preoral appendages, its slender body, less developed swimming legs and the opercular appendage, to the path of development taken by Pterygotus after separation from its common ancestor with Eurypterus. ”

“ Any one who has watched the ease with which this interesting arachnid swims by means of its long spider legs and even overtakes its prey of water insects, can not fail to appreciate the swimming possibilities of even the awkward looking Stylonurus. This water spider also differs from other spiders by having the median eyes raised on a round mound, and the lateral ones on oblique nodes, just as in the eurypterids and especially in Stylonurus. The water spider swims with its right (dorsal) side up, and by analogy we infer that the similar Stylonurus group did the same. ”

“ Paulmier states that "as may be supposed from the finlike posterior legs, it is a good swimmer and may often be found at the surface." On the other hand, he says of the lady crab, which has quite similar swimming legs: "Its usual habitat is the sandy beaches, even the most exposed, when at low water mark it buries itself all but the eyes and antennae and is on the watch for enemies and prey. If disturbed when feeding or if dug out, it disappears with great rapidity, burrowing backward into the sand." ”

“ In summing up the larval characters observed in these immature eurypterids we consider as coenogenetic or purely larval the relatively larger size of the carapace, of the compound eyes and of the swimming legs, and the smaller number of the abdominal segments; as palingenetic and phylogenetic, the approximation of the compound eyes to the margin, the prominence of the ocelli and their tumescences, the lack of differentiation of the abdomen and the smaller size of the telson. ”

“ Omitting for the present the peculiar case of the Eurypterus horizons in the Frankfort shale and the Shawangunk grit and returning to the more typical Eurypterus facies represented by the waterlimes and shales of the Salina formation of western New York, we encounter the venerable and still current view that these beds were formed under conditions of much abnormal salinity. ”

“ The species which seem to us of especial interest in the explanation of the strange structure of the swimming leg are Eurypterus (Onychopterus) kokomoensis and Dolichopterus macrochirus. The former [pl. 26, fig. 2] shows the most primitive shape of the swimming leg known to us. This primitive character manifests itself most distinctly in the following features. Beginning at the distal end, the ninth segment is a well developed terminal claw, such as is found on the other legs, indicating that the minute ninth segment of Eurypterus is a reduced terminal claw. ”

“ From present evidence it may be stated as a general proposition that there are two lines of development of the genital appendages of the operculum in the eurypterids, those of the Pterygotus group and those of the Eurypterus group. In the first, the appendage is composed of but one unpaired lobe, which may become more or less elaborated, as in Slimonia. In the second group two unpaired and two paired lobes are developed. ”

“ From these notes on the habits of the American king crab it may be inferred that though a good swimmer, it mostly frequents the bottom and is a crawler, but feeds by digging in the mud and sand. In an aquarium such as that in Castle Garden, New York, it can be seen to swim easily by napping the powerful sternites which thus function as breathing and swimming organs. ”

“ In a recent exhaustive discussion, Girty [25] has reached the conclusion that it seems most reasonable to regard the fauna of the Carbonic of Pennsylvania "as a natural assemblage of species selected and modified by a habitat, if not in strictly marine, at least not in strictly fresh waters."
There is, however, clear evidence at hand of the fresh-water habitat of the Carbonic eurypterids in other regions. One of these is the occurrence of Eurypterus (Adelophthalmus) granosus Jordan in the coal measures of Saarbrücken. ”

“ In the burrowing operation the forward edge of the anterior shield is pressed downwards and shoved forwards, the two shields being inflected, and the sharp point of the tail providing the fulcrum as it pierces the mud, while underneath the feet are incessantly active, scratching up and pushing out the earth on both sides. There is a singular economy of force in this excavating action, for the alternating doubling up or inflecting and straightening out of the two carapaces with the pushing purchase exerted by the tail spine, accomplish both digging and a subterranean progression. ”

“ An additional point to be brought forward to show that the trilobites probably pass the greater part of their life on their back, and died in that attitude, is that the young Limulus generally feed while turned on their back; moving at an angle with the bottom, the hind extremity raised, they throw out their feet beyond the anterior edge of the carapace, browsing, as it were, upon what they find in their road, and washing away what they do not want by means of a powerful current produced by their abdominal appendages. ”

“ 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. ”

“ HISTORY OF INVESTIGATIONS UPON THE EURYPTERIDS OF NEW YORK AND ADJOINING TERRITORY
The Siluric rocks of New York are, of all countries of the world, the richest in eurypterids. The first example ever described was obtained here. The specimen was found in Westmoreland, Oneida co., and was regarded by Dr S. L. Mitchill [1818]
Figure 1 DeKay's figure of Eurypterus remipes as a fossil fish of the genus Silurus, an error obviously induced by the peculiar catfishlike aspect of the carapace. ”

“ In the group represented by Eusarcus, the carapace is subtriangular, raised in front, where it bears the marginal compound eyes, the walking legs are powerful and provided with long spines, the preabdomen is very broad and flat, the tail long and scorpionlike, with a curved spine that could be raised above the body as an organ of defense. This is a quite aberrant and highly specialized genus and doubtless its habits were different from those of the other genera. ”

“ But aside from this observation, the fact that the attachment of the chelicerae, in Slimonia, Eurypterus and Hughmilleria, has since been fully established to be directly in front of the mouth as in Limulus, leaves little doubt that the large pincers of Pterygotus, if they are at all homologous to the minute chelicerae of those three genera, must have had the same place of insertion, viz, at the posterior end of the epistoma. We may add that the homology of the large pincers of Pterygotus with the chelicerae of Limulus, Eurypterus etc. ”

“ We estimate this diversity in some measure on the striking differences in test sculpture presented and even though this may be an unsafe guide to either specific or generic distinctions yet these sculptures are in so large measure unlike those of better known species that they must be given full worth. These characters are fully elucidated at the proper place in the descriptive part of this book and there are among them undeniable evidences of ornament which we have come to recognize as indicative of the genera Eurypterus, Eusarcus, Hughmilleria and Pterygotus. ”

“ The body of the Eurypterida consists of 18 segments and is functionally divided into two sharply distinct regions, the cephalothorax or prosoma, consisting of six fused segments and the abdomen consisting of 12 segments. The cephalothorax with its appendages is specialized for locomotion and prehension, while the abdomen is essentially a vegetative sac. The six segments of the cephalothorax find their expression in the six pairs of appendages. The cephalothorax consists of the carapace or head shield and the organs of the underside. ”

“ The slender fishlike form is typically expressed by Hughmilleria [see restoration, pl. 59] where the body expands but slightly in the anterior third and then tapers very gradually to the caudal spine; the head shield is semiovoid and the body less depressed than in other genera. Pterygotus is similarly built, but the head shield is more rounded in front and some species are broad and plump. In Eurypterus the lateral expansion or flattening of the body becomes manifest in both head shield and abdomen and the contraction of the body to the caudal portion is more abrupt. ”

“ It is important to determine which of the two forms of eyes is the more primitive. Laurie has argued in favor of the eyes of Pterygotus, representing the first group, basing his view on a comparison with the trilobites (on the assumption that the eurypterids are derived from the trilobites) , the Scorpionidae and Thelyphonidae, "which must be derived from some way down the eurypterid stem." The Lower Siluric fauna is constituted of species with submarginal eyes and this fact seems to favor Laurie's view. ”

“ Nevertheless, there may here exist a difference from that of Pterygotus, for in the other eurypterids no facets have been observed. It is obvious that both Eurypterus and Eusarcus possessed an exteriorly smooth cornea just as Pterygotus; as to the interior of their compound eye, however, several possibilities present themselves: either the lenses were separate from the cornea and thereby lost in fossilization, not being united by the sclera as in the holochroal eyes of the trilobites; or the facets were so feebly
developed as to escape observation. ”

“ While the morphology of these species is not yet wholly clear, the age of the fauna is a factor of chief interest, for the Lower Siluric has hitherto afforded only a few fragments known under the names Echinognathus clevelandi and Megalograptus welchi. With our present knowledge of this assemblage we are entitled to the inference that in a late stage of the Lower Siluric the eurypterids had attained a diversity and an abundance quite as great as in the Upper Siluric. ”

“ Like Dolichopterus the genus Drepanopterus is also derived from Onychopterus or from forms most nearly represented by that genus. The fifth pair of legs has become still more lengthened and its segments are round or tubular instead of flattened into a paddle; and the ninth segment forms a strong claw corresponding to the size of the whole leg. The legs form together a series that increases regularly in length backward, and the first four pairs are spinous. The carapace is large, the body slender and the telson distinctly styliform. ”