THE LOWER SILURIAN, PERIOD. 107 
in later periods, they are pre-eminently characteristic of the 
earlier portion of the Palzeozoic epoch. 
The Ringed Worms (Azmelzdes) 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 
pai 
4 UB 
A\\ 
\\g 
Ye a 
| 
i —S 
} S SY / 
Fig. 47.—Lower Silurian Crustaceans. a, Asaphus tyrannus, Upper Llandeilo; 4, 
Ogygia Buchii, Upper Llandeilo; c, Txtnucleus concentricus, Caradoc; ad, Caryocarts 
Wrightii, Arenig (Skiddaw Slates) ; e, Beyrichia conzplicata, natural size and enlarged, 
Upper Llandeilo and Caradoc; f, Primitia strangulata, Caradoc: g, Head-shield of 
Calymene Blumenbachii, var. brevicapitata, Caradoc; h, Head-shield of Triarthrus 
Becki (Utica Slates), United States; z, Shield of Leferditia Canadensis, var. Yoseph- 
zana, of the natural size, Trenton Limestone, Canada; 7, 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 of Primitia (fig. 47, f), Bey- 
