ARTHROPODA — CRUSTACEA AND DIPLOPODA. 103 
this tribe live in the sea. Among the oldest genera are 
Etyus from the Gault and Cambridge Greensand, and Xanth- 
osia from the Greensand of Warminster and Cambridge. 
Xanthoims is common in the London Clay, and specimens 
are also shown from the Tertiary rocks of Bavaria and China. 
In the British Tertiary series are to be noted remains of a 
spiny Rhachiosoma (Fig. 50), of the swimming crab Fortumis, 
Fig. 50. — A fossil crab, Rhachiosoma bispinosa, of the tribe Cyclometopa, 
Lower Eocene, Portsmouth. (jVfter H. Woodward. Restored from 
specimen in Table-case 20.) 
and of the edible crab Cancer. A very large species of the 
latter comes from Patagonia, and there are some large 
examples of a Scylla from the Philippines. The river-crabs 
are represented by Potamon [^Thdplmsa^ from the freshwater 
Miocene beds of Oen ingen. 
CLA.SS DIPLOPODA (Millipedes). 
The millipedes of to-day are inhabitants of the land, with 
a distinct head and a worm-like body of many similar seg- 
ments, each enclosed in a horny ring. Many of the segments 
bear two pairs of legs apiece, and thus represent two primitive 
segments fused together. In some millipedes the under part 
of the ring still consists of two plates, one to each pair of 
legs. In many Palaeozoic millipedes not only does each 
segment bear two pairs of legs, but, both on the back and on 
the under side, the ring is composed of two plates. On the 
side of each .segment, near the attachment of the legs, is 
an opening. These openings, called stigmata, lead to the 
tracheae or air-tubes by which the animal breathes. The 
head bears two clusters of eyes and a pair of short feelers 
or antennae, and is furnished underneath with two pairs of 
Gallery 
VIII. 
Table-ease 
20 . 
Wall-ease 
12c. 
