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 niarine 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 asin 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 iscommonly termed 
the swimming foot or ‘‘ paddle,” but 
Laurie has suggested that the animal 
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 
etiam! lobe attached to them; the latter 
a y HT y , varies in form in the same species, 
/ acs f correlating with sex. Underlying 
eae the operculum are a number of leaf- 
VI like structures, which are almost 
certainly branchial in function and 
represent the reduced appendages ot 
the second free segment. The re- 
maining abdominal appendages are plate-like and bear 
branchiae on their posterior surfaces, being in a general 
way comparable to those of Limulus (Laurie). 


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 
Fia. 1417. elongate, narrow, attaining some- 
Eurypterus Fischeri, Eichw. Silurian; Rootzikiill, Island of times a length of 50 em. Cephalo- 
Ouse], “Restoration aftr Schmidt ffs yatoral sue, 4, Dorel! thorax contained five or six ties 
1—13 abdominal segments. B, Basipodites of the first pair of legs 1m total length of the body, 
(D;,the antennae () between inem now known to be cheliceafe. depressed convex, sub-quadrate, 
a, Median process composed of four joints (ai—4); b, Lateral with rounded anterior angles. 
portions ; ¢, Triangular piece at the base of the first moiety of the ~ : . 
median process (after F. Schmidt), Anterior margin nearly straight, 
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 

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