4 62 



DISTRIBUTION IN SPACE AND TIME 



been related to the Crustaceans. The King-Crabs (Xiphosurd), 

 now represented by a single genus (Ltmu/uf), first made their 

 appearance in Palaeozoic times. They are sometimes included 

 with the Eurypterids in a special group {Merostomata). 



Centipedes and Millipedes (Myriapodd) were represented by 

 several palaeozoic forms with somewhat primitive characters, while 

 among Spider -like Animals (Arachnidd) there were Scorpions, 

 which appeared comparatively early, Whip - Scorpions, and 

 Spiders, besides representatives (e.g. Eophrynus, fig. 1318) of 



Fig. 1317. A Eurypterid (Pterygotus], much reduced Fig. 1318. An extinct Arachnid (Eophrynus) 



an order limited to the epoch. Four orders of Insects (Insectd) 

 had palaeozoic representatives, i.e. Primitive Wingless Insects 

 (Apterd), Straight- Wings (Orthopterd), Net- Wings (Neuroptera), 

 and Bugs (ffemipterd). Some of them were of considerable size, 

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

 to existing orders. 



PAL/EOZOIC MOLLUSCS (MOLLUSCA). Among Head- Footed 

 Molluscs (Cephalopoda] now existing, only the Pearly Nautilus 

 (Nautilus) 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 



