THE LOWER SILURIAN PERIOD. 



109 



commenced their existence, so far as known, in the Upper 

 Cambrian ; and though examples are not absolutely unknown 

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

 earlier portion of the Palaeozoic epoch. 



The Ringed Worms (Annclides) 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 

 group of the Crustaceans. Amongst these are numerous little 

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

 richia (fig. 47, <?), and Leperditia (fig. 47, i and /). Most of 

 these are very small, varying from the size of a pin's head up 

 to that of a hemp seed; but they are sometimes as large as 

 a small bean (fig. 47, /)> an d they are commonly found in 

 myriads together in the rock. As before said, they belong to 

 the same great group as the living Water-fleas (Ostracoda}. 

 Besides these, we find the pod-shaped head-shields of the 

 shrimp-like Phyllopods such as Caryocaris (fig. 47, d) and 

 Ceratiocaris. More important, however, than any of these are 

 the Trilobites, which may be considered as attaining their maxi- 



Fig. i&.Ptilodictya falciformis. a, 

 Small specimen of the natural size ; fe, 



Fig. 49. A, Ptttodlctya acuta ; B, Ptil- 

 odictya Schnfferi. a. Fragment, of the 



Cross-section, showing the shape of the natural size ; 6, portion, enlarged to 



front; c. Portion of the surface, en- show the cells. Cincinnati Group of Ohio 



larged. Trenton Limestone and Cincin- and Canada. (Original.) 

 nati Group, America. (Original.) 



mum development in the Lower Silurian. The huge Paradoxides 

 of the Cambrian have now disappeared, and with them almost 

 all the principal and characteristic " primordial " genera, save 

 Olenus and Agnostus. In their place we have a great number 



