stern coasts, and on the Asiatic coast of the
Pacific. The Atlantic species (_L. polyphemus_) is common on the
coasts of the United States, and is known as the king-crab or
horse-shoe crab. A single specimen was found in the harbour of
Copenhagen in the 18th century, having presumably been carried over by
a ship to which it clung.

A species of _Limulus_ is found in the Buntersandstein of the Vosges;
_L. Walchi_ is abundant in the Oolitic lithographic slates of Bavaria.

[Illustration: FIG. 45.--Ventral view of _Limulus polyphemus_.

1 to 6, The six prosomatic pairs of appendages.
abd, the solid opisthosomatic carapace.
tels, the post-anal spine (not the telson as the lettering would
seem to imply, but only its post-anal portion).
operc, the fused first pair of mesosomatic appendages forming the
genital operculum.

(From Parker and Haswell, _Text book of Zoology_, after Leuckart.)]

The genera _Belinurus_, _Aglaspis_, _Prestwichia_, _Hemiaspis_ and
_Bunodes_ consist of small forms which occur in Palaeozoic rocks. In
none of them are the appendages known, but in the form of the two
carapaces and the presence of free somites they are distinctly
intermediate between _Limulus_ and the Trilobitae. The young form of
_Limulus_ itself (fig. 40) is also similar to a Trilobite so far as
its segmentation and trilobation are concerned. The lateral eyes of
_Limulus_ appear to be identical in structure and position with those
of certain Trilobitae.

[Illustration: FIG. 46.--_Eurypterus Fischeri_, Eichwald. Silurian of
Rootzikil. Restoration after Schmidt. The dorsal aspect is presented
showing the prosomatic shield with paired compound eyes and the
prosomatic appendages II. to VI. The small first pair of appendages is
concealed from view by the carapace, 1 to 12 are the somites of the
opisthosoma; 13, the post-anal spine.

(From Zittel's _Text-book of Palaeontology_, The Macmillan Co, New
York, 1896.)]

Order 2. Gigantostraca (figs. 46, 47).--Free-swimming forms, with the
appendages of the 6th or 5th and 6th pairs flattened or lengthened to
act as oars; segments of mesosoma and metasoma (= opisthosoma), twelve
in number.

Appendages of anterior pair very large and chelate.

Sub-order Pterygotomorpha, Pterygotidae (_Pterygotus_).

Appendages of anterior pair minute and chelate.

/ Stylonuridae (