Recent Additions to our Knowledge of the

Eurypterida.

INCE the publication of Dr. Henry Woodward's "Monograph of important papers dealing with this group has been comparatively small. The additions to our knowledge of the true structure of these interesting Palæozoic forms have, however, been of great value, and their zoological affinities may be said to be fairly well established.

The classical works of Huxley and Salter (1), Hall (2), and Woodward demonstrated that the Eurypterids were Arthropods. whose body consisted of a carapace followed by twelve free segments and terminating in a telson, the surface being more or less covered with scale-like markings. The under surface of the carapace bore a number of legs-five pairs, according to the above-cited authorities, and it is in connection with these that some of the most important additions to our knowledge have been made. These legs consist typically of a basal joint, the inner margin of which is armed with teeth and serves as a jaw, much as in Limulus or Apus. At the posterior end of the tooth-bearing margin is attached (6) a small oval epicoxite exactly similar to that found in the same place in Limulus, and at the anterior angle is attached the long, usually sixjointed ambulatory appendage.

The last pair of legs is characterised in all members of the group, except Stylonurus, by its greater size and usually somewhat flattened form, and ends in an oval plate. This is usually termed the swimming foot, but it seems more probable that it was used for anchoring the animal firmly in the soft mud of the sea bottom, and possibly also for shovelling up the sand and mud when the beast wished to conceal

itself.

In Pterygotus the most anterior pair of appendages are large pincers, probably prehensile in function, and evidently attached in front of the mouth, as jaw bases are wanting. They seem to have consisted of only three long joints, though owing to the crumpling of the cuticle of the proximal joint there is often an appearance of a greater number. Behind the mouth there are-according to Schmidt (4)four pairs of ordinary walking legs, followed by the large "swimming

feet." This would make the whole number of appendages six instead. of five, as described by Huxley, who only recognised three pairs of walking legs.

In Slimonia and Eurypterus the full complement of post-oral appendages (five pairs) has long been recognised. In the former the first pair, described by Woodward (3), and by him termed "antennæ," is modified to form tactile organs, but is undoubtedly post-oral in position, as the basal joint bears teeth. Pre-oral appendages have been described in Eurypterus fischeri by Schmidt (4) as a pair of fine antenniform structures. In Slimonia and in Eurypterus scorpioides and E. conicus the pre-oral appendages have been described (6) as small stout pincers, much like the cheliceræ in Limulus, and corresponding generally to the big pincers in Pterygotus, though probably masticatory in function rather than prehensile. The difference between these appendages as described in E. fischeri and E. scorpioides, etc., is very great, but we must wait for further evidence before making the apparently logical change of dividing Eurypterus into two distinct. genera, or even families. There would be a certain awkwardness in founding one's classification on a structure which has only been described in three species out of the forty or fifty in the genus.

The legs in the other genera of Eurypterida are less well-known. In Stylonurus the last two pairs are enormously elongated. The other legs are only known by fragments, but among the fragments figured in a recent paper by Hall and Clark (5) is a well-marked pair of cheliceræ, which were no doubt pre-oral in position. Limbs intermediate in form between the elongated type of Stylonurus and the broad "swimming feet" of Eurypterus have been described in Dolichopterus (2) and Drepanopterus (8).

. The hard parts, other than appendages, on the ventral surface of the carapace consisted of the epistoma in front of the mouth and the metastoma behind it. The epistoma was described by Huxley and Salter in Pterygotus, but they were misled by the direction of the scale markings on it, and tried-as pointed out by Schmidt-to get it in hind side before. It consists in Pterygotus, in which it is best known, of a thin, semicircular plate, which covered the space between the anterior margin of the carapace and the mouth, the pre-oral appendages being attached close to its posterior border. The reversed position of the scale markings, which have their convex side directed forward, would seem to point to its being morphologically the inturned front border of the carapace. It probably corresponds to the hypostoma of Trilobites, which occupies the same position.

The metastoma is a heart-shaped plate attached along the middle line to the ventral wall of the body, between the bases of the last pair of legs and extending outwards and forwards so as to enclose the jaws in a kind of chamber.

The margins of the carapace are often bent over on to the ventral surface to a greater or less extent. This is particularly well-marked