674 



ARTHROPODA 



SUB-KINGDOM VII 



the Ordovician of Bohemia and North America ; with marine Crustacea (Phyllocarids and 

 Ostracods) in the Silurian ; with Ostracoderms and Arthrodites in the Devonian ; and with 

 land plants, scorpions, insects, fishes, and fresh-water amphibians in the productive Coal 

 Measures. It is apparent, therefore, that from being originally marine forms, they became 

 gradually adapted to brackish, and possibly even fresh-water conditions. It has been 

 suggested that certain clusters of small roe-shaped bodies may be the ova of Pterygotus, 



On the under side of the cephalothorax there are five pairs of postoral appendages. 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 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 six-jointed ambulatory 

 appendage. The last pair of legs is charactised in all members of the order except Stylonurus, 



by its greater size and usually some- 

 what flattened form, and ends in an 



. i oval plate. This is commonly termed 



the swimming foot or " paddle," but 



Laurie has suggested that the animal 



TV ^^-^^^JSiilKJJBKSgF^^ -*& used it for anchoring or burying 



itself in the mud. 



The ventral surface of the first 

 two abdominal segments is covered 

 by the genital operculum, which con- 

 sists of a pair of plates meeting in 

 the middle line and having a median 

 lobe attached to them ; the latter 

 varies in form in the same species, 

 correlating with sex. Underlying 

 the operculum are a number of leaf- 

 Hke structures, which are almost 

 certainly branchial in function and 

 represent the reduced appendages ol 



7 \ IHt^* 1 ^ tkV* jj the second free segment. The re- 



maining abdominal appendages are plate -like and bear 

 branchiae on their posterior surfaces, being in a general 

 9 \TOlf 1 / wa y com P ara ^ e t those of Limulus (Laurie). 



i A I:Bi s ffff IK / 



Family 1. Eurypteridae. Burmeister. 



This family, in the present condition of our 

 knowledge, is allowed to embrace all the members of 

 the order, with characters as given above. Future 

 investigation will undoubtedly show reason for the 

 establishment of other families, 

 and some authors have already 

 been disposed to separate Ptery- 

 gotus into a distinct group. 



Eurypterus, Dekay (Lepido- 

 derma, Reuss; (?) Campylocephalus, 

 Eichw.), (Figs. 1417-1421). Body 



Flo< 1417> elongate, narrow, attaining some- 



Eurypterrus Fischeri, Eichw. Silurian ; Rootzikull, Island of times a length of 50 cm. Cephalo- 

 Oesel. Restoration alter Schmidt. 1/5 natural size. A, Dorsal thorax contained five or six times 

 aspect, showing legs II. VI. beneath the cephalothorax, and . . ,, , , 



113 abdominal segments. B, Basipodites of the first pair of legs in total length OI tne body, 

 (7); the antennae (a) between them now known to be chelicerate. r i or . TOO< WI r-nnvpY snb-nuadrate 

 C, First lamellar appendage (operculum) of the ventral side, i/j. Depressed convex, E lie, 



a, Median process composed of four joints (a 1 4 ); ft, Lateral with rounded anterior angles, 

 portions ; c, Triangular piece at the base of the first moiety of the . . ,,. noarlv traio-lit 



median process (after F. Schmidt). Anterior margin nearly straignt, 



posterior slightly concave. Eyes 



reniform, somewhat in front of the middle ; between them and close to the axial line 

 are two ocelli. The entire cephalothorax bordered by a narrow marginal furrow, 

 and the margin broadly enfolded on the ventral side. In the middle of the lower side 



