ARTHEOPODA — CRUSTACEA. 



103 



Etyits from the Gault and Cambridge Greensand, and Xant- Gallery 

 liosia from the Greensand of Warminster and Cambrido^e. ^ y,-^-^-''" 

 Xanthopsis is common in the London Clay, and specimens 20. 

 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 Forhmus, 



and of the edible crab Cancer. A very large species of the 



latter comes from Patagonia, and there are some large Wall-ease 



examples of a Scylla from the Philippines. The river-crabs ^^c. 



are represented by Theliohusa from the freshwater Miocene 



beds of Oeningen. 



Class 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 



