THE LOWER SILURIAN PERIOD. 



107 



in later periods, they are pre-eminently characteristic of the 

 earlier portion of the Palaeozoic epoch. 



The Ringed Worms (Annelides) are abundantly represented 

 in the Lower Silurian, but principally by tracks and burrows 

 similar in essential respects to those which occur so commonly 

 in the Cambrian formation, and calling for no special com- 

 ment. Much more important are the Articulate animals, rep- 

 resented, as heretofore, wholly by the remains of the aquatic 



Fig. 47. Lower Silurian Crustaceans, a, Asa/thus tyranntts, Upper Llandeilo; />, 

 Ogygia Bnchii, Upper Llandeilo ; c, Trimidens conccntricus, Caradoc ; if, Carypcaris 

 H 'ris/iiii, Arenig (Skicldaw Slates) ; e, Beyrichia compUcata, natural size and enlarged. 

 Upper Llandeilo and Caradoc;/, Ptimitia stranmlata. Caradoc: , Head-shield of 

 Calymene Blumenbachii, var. brevicapitata, Caradoc ; h, Head-shield of Triarthnts 

 Becki (Utica Slates), United States ; i, Shield of Lefierditia Canadensis, var. Josef h- 

 ifina, of the natural size, Trenton Limestone, Canada ; /, The same, viewed from the 

 front. (After Salter, M'Coy, Rupert Jones, and Dana.) 



group of the Crustaceans. Amongst these are numerous little 

 bivalved forms such as species Q{ Primitia (fig. 47, /), Bey- 



