Page:The Eurypterida of New York Volume 1.pdf/416

 we place the Limulava with them their definition has to be completely changed.

In the Eurypterida the cephalothorax bears six pairs of appendages, the first of which are chelate antennae or chelicera; these are apparently absent in, where there are only five pairs, the first of which consists of simple antennae. In Slimonia the corresponding pair of legs is also tactile and the antennae of Sidneyia apparently bore coxae, so that the character of the first pair of legs would not constitute a difference of more than generic value. Chelicera, however, are characteristic of the Eurypterida, and if Sidneyia is an eurypterid, it is to be assumed that these, often delicate organs, will be found on further research. The third pair of legs is one of the remarkable features of this organism. It is described as "multichelate," or as possessing "broad, strong joints in small specimens, and in large specimens it is developed into peculiarly constructed and complex chelate terminal sections; this is formed of twelve or more joints of a forward curving appendage to which are attached on the anterior side long spines carrying numerous smaller spines on the margin opposed to the main body of the appendage." If indeed the leg consists of two jointed appendages, as reproduced in op. cit. pl. 4, figures 1 and 4, then we have here a biramous leg such as would be characteristic of the Crustacea in contrast to the Merostomata, and a feature which indicates that the fossil is a crustacean rather than an arachnid. The long bladelike appendages resemble those of the legs of  and   and might, like the latter, have served to make a swimming organ of the third pair of legs. There is still a bare possibility that they are analogous to the male clasping organ observed by Holm on the second walking leg of  and which had a sexual function. However that may be, these complex legs of Sidneyia are important in showing a certain advanced adaptation of the genus in the form of these organs. The last pair is described as short and bearing an outer, broad joint or palp fringed with five branchial setae or spines. This is another character entirely foreign to the arachnids and ascribable to the crustaceans.