SILURIAN PERIOD 35 



regards habits they had parted company ; one section con- 

 sisting of more or less active swimmers ; the other of forms, 

 capable indeed of swimming, but more or less addicted to 

 prowling about the sea-floor. To judge from their greater 

 size, and variety, the active swimmers were the more prosper- 

 ous. The last pair of limbs or appendages, in their case, had 

 been modified into powerful paddles, and the tail was some- 

 what short, and in some instances plate-like (Eurypterus, 

 Pterygotus). In the prowling forms the hind-hmbs were 

 greatly elongated and single-clawed, and the tail was long 

 and slender (Stylonurus). Sea-scorpions brought forth 

 monsters in later times ; but the largest known Silurian form 

 scarcely attained two feet in length. 



Animals intimately connected with sea-scorpions, and SCORPIONS 

 probably an off-shoot of them, were now coming into view 

 (PalcBophonus). These creatures seem to have closely re- 

 sembled the scorpions of our own time ; excepting that the 

 limbs terminated not with double but single claws. They 

 certainly possessed venomous stings ; but it is doubtful if 

 they resembled living scorpions in being land-dwellers. 

 They may well, however, have paid occasional visits to the 

 leafy scenes, where an increasing insect-population must have 

 presented an attraction. 



Ostracods seem to have had a set-back after their great ostracods 

 burst of prosperity in the previous Period. Small crustacean 

 life, however, was now being enriched by the appearance of 

 amphipods, popularly known as " beach fleas " and " sand- amphipods 

 hoppers " (Necrogammarus). 



Through the greater part of the Silurian there is no satis- 

 factory evidence of any animals other than invertebrates ; 

 but towards the close of the Period there is little doubt that 

 animals with at least rudimentary backbones had emerged 

 from invertebrate ranks. 



The great majority of these representatives of higher ostraco- 

 life, although fish-hke in appearance, were much of the grade derms 



of hags and lampreys — animals not regarded as true fishes. 

 Some, measuring about seven inches in length, had flattened 

 heads, big in comparison with the body, and were somewhat 

 skate-like in shape. Their skins, Hke the skins of sharks, 



