SUB-CLASS MEROSTOMATA (GIGANTOSTRACA) Differing from the Limulava in having prosoma composed of six somites, the preoral appendages not antenniform, the append ages of the opisthosoma, of which there are only six pairs, lami nate and not locomotor in function, the terminal somite not ex panded into a caudal fin but with a postanal spine or plate.
Order Xiphosura. Trilobite-like forms with expanded pleural areas, the dorsal surface generally "trilobate," the opisthosoma with nine or ten somites and its appendages, where known, biramous.
Most of the genera of this order are Palaeozoic and extinct. They differ from the Mesozoic and existing king crabs (Limulidae) in having the opisthosoma distinctly segmented. Well defined genera are Belinurus, Hemiaspis, Pseudoniscus, and others; but their classification needs revision. The discovery in these that some genera related to Agalaspius, formerly regarded as closely akin to these forms, have Trilobite-like appendages and other characters may show that these ancient forms are not so nearly related to the Limulidae as has been supposed.
Order Eurypterida. Chiefly differing from the Xiphosura in be ing typically scorpion-like in shape, without pleural areas, with the opisthosoma composed of i 2 somites of which the posterior six are limbless and the anterior six provided with unbranched plate-like appendages, the first carrying a median rod which prob ably acted as an ovipositor in the female, as a penis in the male.
The Eurypterida occur in Palaeozoic strata and are wholly ex tinct. Some of them, being over two yards in length, are the largest arthropods known. They exhibit great variation in the structure of their prosomatic limbs, the last pair being frequently modified as broad paddles (Eurypterus, Pterygotus), or the last two pairs may be very long and act as oars (Stylomurus) . The preoral appendages may be long, massive pincers (Pterygotus) or insignificant in size. By these and other characters the group is divisible into a number of families and sub-families.