LOWER PALAEOZOIC FAUNAS 153 



as to whether certain small fossils are detached Cirripede 

 plates or primitive Pelecypod shells. The curious, fir-cone- 

 like Turrilepas is known from the Cambrian onwards, and 

 seems to have disappeared in Devonian times. 



Lower Palaeozoic Malacostraca all belonged to the 

 primitive series Phyllocarida, usually showing super- 

 ficial resemblance to " shrimps." Hymenocaris from the 

 Cambrian, and Ceratiocaris (PL x. fig. 9) from the two 

 succeeding periods, are the best known British repre- 

 sentatives of the group. 



Arachnids of the era belonged to the orders Synxi- 

 phosura, Eurypterida and Scorpionida. The first- 

 named group was restricted to the Lower Palaeozoic, 

 and included Trilobite-like forms that were probably 

 ancestral to the Limulus -series and Eurypterids. That 

 order is known from the Palaeozoic only, but ranged 

 from the Cambrian to the Permian, with a marked 

 acme in Silurian and Devonian times. Beltina, a prob- 

 lematical Eurypterid from the pre-Cambrian, is of 

 doubtful systematic position. The two most familiar 

 members of the order are Eurypterus and Pterygotus. 

 The former genus lingered to the Permian, but was 

 most prominent in the Upper Silurian. The classic 

 locality for its occurrence is in the Isle of Oesel, whence 

 specimens have been obtained from the fine Silurian 

 marls in such perfect preservation that they can be 

 detached from their matrix and mounted for micro- 

 scopic study like recent material. Pterygotus, a gigantic 

 form with peculiar outward similarity to certain con- 

 temporary Antiarchi (e.g. Pterichthys}> appeared with 

 Eurypterus in the Ordovician, but is best known in 

 Britain from the Old Red Sandstone. Scorpions are 

 found in the Upper Silurian of Scotland and elsewhere ; 

 they belong to a distinct family, and their order was 

 not fully differentiated until the Carboniferous period. 



