CRUSTACEA. DECAPODA 331 



strong, with large chelae ; second and third pairs short ; fourth and 

 fifth slender. Abdomen of six segments and a telson in both sexes. 

 Eocene to present day. Ex. D. lamarcki, London Clay. 



Palaeocorystes. Cephalothorax much longer than broad, 

 tapering posteriorly, anterior border not dentate ; rostrum short. 

 Orbital notches large with two small fissures. Cervical suture well 

 defined. The five anterior segments of the abdomen short, the sixth 

 quadrangular. Gault and Eocene. Ex. P. stokesi, Gault. 



Eucorystes. Cephalothorax trapezoidal ; anterior part with 

 tortuous, band-like elevations ; posterior part smooth or finely 

 granular. Cambridge Greensand. Ex. E. carteri. 



Necrocarcinus. Cephalothorax rounded, separated into 



regions by distinct grooves, ornamented with a few prominent 



tubercles. Eostrum triangular. Orbital notches rounded, open 



above, with two small fissures. Gault to Chalk. Ex. N. bechei, 

 Cambridge Greensand. 



Xanthopsis. Cephalothorax rounded, convex, surface punc- 

 tate, the posterior portion with rounded elevations ; the frontal 

 border with four, and the anterior laterals with one to three, tooth- 

 like processes. Orbital notches deep, without fissures. Chelae 

 unequal. Abdomen of the male narrow and formed of four seg- 

 ments and a telson. Abdomen of female broad, composed of six 

 segments and a telson. Eocene. Ex. X. leachi, London Clay. 



Distribution of the Decajwda 



Most of the Decapoda are marine, the larger number 

 living in shallow water; amongst those which inhabit 

 deep water are representatives of the Eryonidse — a family 

 which flourished in Jurassic times. Some groups of the 

 Macrura and Brachyura live in fresh water, whilst some of 

 the Anomura and Brachyura are terrestrial. 



No undoubted examples of the Decapoda are known 

 to occur in Palaeozoic deposits ; but representatives be- 



