_Stylonurus_).
Sub-order Eurypteromorpha < Eurypteridae (_Eurypterus_,
\ _Slimonia_).

[Illustration: From Zittel's _Palaeontology_.

FIG. 47.--_Pterygotus osiliensis_, Schmidt. Silurian of Rootzikil.
Restoration of the ventral surface, about a third natural size, after
Schmidt.

a, Camerostome or epistoma.
m, Chilarium or metasternite of the prosoma (so-called metastoma).
oc, The compound eyes.
1 to 8, Segments of the sixth prosomatic appendage.
I' to V', First five opisthosomatic somites.
7', Sixth opisthosomatic somite.

[Observe the powerful gnathobases of the sixth pair of prosomatic
limbs and the median plates behind m. The dotted line on somite I
indicates the position of the genital operculum which was probably
provided with branchial lamellae.]]

_Remarks._--The Gigantostraca are frequently spoken of as "the
Eurypterines." Not more than thirty species are known. They became
extinct in Palaeozoic times, and are chiefly found in the Upper
Silurian, though extending upwards as far as the Carboniferous. They
may be regarded as "macrourous" Xiphosura; that is to say, Xiphosura
in which the nomomeristic number of eighteen well-developed somites is
present and the posterior ones form a long tail-like region of the
body. There still appears to be some doubt whether in the sub-order
Eurypteromorpha the first pair of prosomatic appendages (fig. 46) is
atrophied, or whether, if present, it has the form of a pair of
tactile palps or of minute chelae. Though there are indications of
lamelliform respiratory appendages on mesosomatic somites following
that bearing the genital operculum, we cannot be said to have any
proper knowledge as to such appendages, and further evidence with
regard to them is much to be desired. (For literature see Zittel,
22*.)

Grade b (of the Eu-arachnida). EMBOLOBRANCHIA (Aeropneustea).

In primitive forms the respiratory lamellae of the appendages of the
3rd, 4th, 5th and 6th, or of the 1st and 2nd mesosomatic somites are
sunk beneath the surface of the body, and become adapted to breathe
atmospheric oxygen, forming the leaves of the so-called lung-books. In
specialized forms these pulmonary sacs are wholly or partly replaced
by tracheal tubes. The appendages of the mesosoma generally
suppressed; in the more primitive forms one or two pairs may be
re