218 PALEONTOLOGY 



The Decapoda Macrura (lobsters, etc.) occur in 

 various formations from the Triassic upwards. Well- 

 preserved specimens are abundant in the Lithographic 

 stone of Solnhofen, Bavaria. 



The Decapoda Brachyura (crabs) first appear in the 

 Middle Jurassic. A species, Pal^ocorystes stokest, abounds 

 in one bed in the Gault and (as a derived fossil) in the 

 Cambridge Greensand. Other forms occur in phosphatic 

 nodules in the London Clay. 



The Eurypterida had the body divided into head, 

 mesosoma, and metasoma, each of six somites. The 

 head-appendages all bear gnathobases, none taking 

 the character of antennae (a fundamental distinction 

 between Arachnida and Crustacea) ; they increase in 

 size backwards, the last being a very large pair of claws. 

 There are no walking-limbs behind the head, the appen- 

 dages of the mesosoma being broad and flat, and bearing 

 book-like gills, while the metasoma has no appendages. 

 Behind the eighteenth somite is a telson, which varies 

 in character from an expanded swimming organ (Ptery- 

 gottis) to a long and narrow rod (Eiirypterus). In early 

 Palaeozoic times this order seems to have been confined 

 to American seas, but towards the end of the Silurian 

 period it spread over the European area, giving rise to 

 some gigantic forms (Pterygohts, a nectic form, two metres 

 long ; Stylonurus, benthic, one metre). Some of the 

 Devonian and later eurypterids became adapted to a 

 freshwater habitat, the last of these being found in the 

 Permian, after which the order was extinct. 



The Xiphosura are close allies of the eurypterids, 

 differing from them in the fusion of the somites of the 

 meso- and meta-soma into one piece. They range from 

 Cambrian to Recent, and are mainly marine, but in the 

 later Palaeozoic some freshwater forms existed. Some 

 of the earlier genera have a close external resemblance to 



