332 CRUSTACEA. DECAPODA 



longing to the section Macrura (e.g. Pemphix), appear 

 in the Trias. The Macrura become abundant in the 

 Jurassic, where, amongst others, the genera Glyphea, 

 Eryon, Mecochirus, uEger, and Eryma are found. In the 

 Cretaceous, Enoploclytia, Hoploparia and Meyeria occur. 

 Podocrates is found in the Eocene, and Propalwmon in 

 the Oligocene deposits. 



The first undoubted examples of the Brachyura are 

 found in the Jurassic rocks, but only two or three genera 

 are represented, of which Prosopon appears first in the 

 Inferior Oolite and survives until the Lower Cretaceous, 

 whilst Palceinachus is found only in the Forest Marble. 

 In the Cretaceous the Brachyura become more abundant 

 and are represented by Palceocorystes, Eucorystes, Ne- 

 crocarcinus and several other genera. In the Eocene 

 numerous forms occur, Xanthopsis and Dromia being 

 common. The Brachyura attain their maximum at the 

 present day. 



CLASS III. MYRIAPODA 



The Myriapoda include the millipedes, centipedes, and 

 allied forms. The body consists of a distinctly-marked 

 head, followed by segments which are usually numerous 

 and similar in form, so that, externally, the limits of the 

 thorax and abdomen cannot be defined. The head bears 

 one pair of antennse ; and also mandibles and maxillae. 

 The segments behind the head (except the last) bear in 

 some cases one, in others two, pairs of legs each ; in the 

 latter the segments are really double. The Myriapods 

 breathe by means of trachea?. 



In the fossil state Myriapods are rare. The two 

 principal Orders are : — (1) the Chilopoda, or centipedes, 



