462 



DISTRIBUTION IN SPACE AND TIME 



been related to the Crustaceans. The King-Crabs [Xiphosura), 

 now represented by a single genus [Limiihis), first made their 

 appearance in PaLneozoic times. They are sometimes included 

 with the Eurypterids in a special group {^Merostomata). 



Centipedes and Millipedes [Myriapoda] were represented by 

 several palaeozoic forms with somewhat primitive characters, while 

 among Spider- like Animals [Arachnida) there were Scorpions, 

 which appeared comparatively early, Whip - Scorpions, and 

 Spiders, besides representatives (e.g. Eopluynus, fig. 13 18) of 



Fig. 1317. — A Euryplerid ' Ptcry^^otns), much reduced 



Fig. T31S.— An extinct Arachnid ■ Eo/'ftrynits) 



an order limited to the epoch. Four orders of Insects [liisectd) 

 had pakeozoic representatives, i.e. Primitive Wingless Insects 

 {Aptej-a), Straight-Wings (prthopteni), Net-Wings {Nenropterd), 

 and Bugs [Ilemipterd). Some of them were of considerable size, 

 and it is by no means certain that the older types really belonged 

 to existing orders. 



Pal.eoz(;ic Molluscs (Mollusca). — Among Head- Footed 

 Molluscs [Cephalopoda) now existing, only the Pearly Nautilus 

 [Nanlilus) possesses an external shell, of which the part not 

 occupied by the animal is divided by partitions into a series of 

 gas-filled chambers. But in the palaeozoic rocks we find the re- 

 mains of a great many species thus characterized. Their shells 

 were straight, curved, open spirals, or closed spirals, like that of 

 Nautilus, which dates back to the middle of the epoch. It seems 



