The front part of its body was entirely enveloped in a kind of shield, called a carapace, bearing near the centre minute eyes, which probably were useless, and at the corners two large compound eyes, made up of numerous little lenses, such as we see in the eye of a dragon-fly. This is clearly proved by certain well-preserved specimens. There are five pairs of appendages, all attached under or near the head. Behind the head follow thirteen rings, or segments, the last of which forms the tail, two at least of these bore gills for breathing. All but two of them, below the mouth, must have been beautifully articulated, so as to allow them to move freely, as we see in the lobster of the present day. But look at that lowest and largest pair of appendages, the end joints of which are flattened out, and you will see that they must have been a powerful oar-like apparatus for swimming forwards. We can fancy this creature propelling itself much in the same way as a "water-beetle" rows itself through the water in a pond. In all other crustaceans the antennæ are used for feeling about, but in the Pterygotus they are used as claws for seizing the prey. In general external appearance, this huge Pterygotus greatly reminds us of a tiny fresh-water crustacean, known as Cyclops--because it has only one eye, like the giant in Homer's _Odyssey_. This little creature, which is only 1/16 inch in length, is an inhabitant of ponds. From its large eyes, powerful oar-like limbs, or appendages, and from the general form of its body, Dr. Henry Woodward (the author of a learned monograph on these creatures) concludes that the Pterygotus was a very active animal; and the reader will easily gather from its pair of antennæ, converted at their extremities into nippers, and from the nature of its "jaw-feet," that the creature was a hungry and predaceous monster, seizing everything eatable that came in its way. The whole family to which it belongs--including Pterygotus, Eurypterus, Slimonia, Stylonurus, and others--seems to have been fitted for rather rapid motion, if we may judge from the long tapering and well-articulated body. In two forms (Pterygotus and Slimonia) the tail-flap probably served both as a powerful propeller, and as a rudder for directing the creature's course; but others, such as Eurypterus and Stylonurus, had long sword-like tails, which may have assisted them to burrow into the sand, in the same way that king-crabs do. Eurypterus remipes is shown in Fig. 2.