CRUSTACEA. DECAPODA. 193 



Xanthopsis. Cephalothorax rounded, convex, surface 

 punctate, the posterior portion with rounded elevations; 

 the frontal border with four, and the anterior-laterals 

 with one to three, tooth-like processes. Orbital cavities 

 deep, without fissures. Chelae unequal. Abdomen of the 

 male narrow and formed of five segments, owing to the 

 fusion of the third, fourth, and fifth segments. Abdomen 

 of female broad, composed of seven segments. London 

 Clay. 



Distribution of the Decapoda. 



All the Brachyura, and most of the Macrura, are 

 marine. The latter are the first to appear in the geo- 

 logical series, a few genera being found in the Upper 

 Palaeozoic ; the earliest is Palceopalcemon from the Upper 

 Devonian; in the Carboniferous Anthrapalcemon and 

 others occur. The section becomes much more abundant 

 in the Mesozoic, attaining its maximum in the Jurassic, 

 where we get, amongst others, the genera Glyphcea, Eryon, 

 jEger, and Eryma. In the Cretaceous, Enoploclytia, 

 Hoploparia, and Meyeria occur. Only a few forms are 

 found in the Tertiary. The Brachyura are represented 

 by one or two genera only in the Jurassic ; in the Creta- 

 ceous by Paloeocorystes and several others; the Eocene 

 forms are much more numerous, Xanthopsis being the 

 commonest genus ; but the section attains its maximum 

 at the present day. 



ORDER, ARTHROSTRACA. 



There is no cephalothoracic shield ; there are usually 



seven (sometimes fewer) segments in the thorax ; and 



usually six in the abdomen, and a telson, but the abdomen 



is sometimes rudimentary. The head carries two pairs of 



w. P. 13 



