126 INVERTEBRATE PALAEONTOLOGY 



The forms of life predominant during the era were 

 the Graptolites, Brachiopods and Trilobites. The first- 

 named group arose in the later stages of the Cambrian, 

 and became practically extinct before the end of the 

 Silurian. During its short career it attained a very 

 prominent acme ; and by its rapid and abundant 

 variation provided satisfactory zonal indices. Brief 

 search in any Ordovician or Silurian shales usually 

 reveals innumerable remains of Graptolites. The 

 Brachiopods belonged chiefly to the three orders 

 Atremata, Neotremata and Protremata. Of these, the 

 two first were more abundant and varied during the 

 Lower Palaeozoic than at any later stage; but they 

 were overshadowed by the acmaic exuberance of the 

 Orthacea, Strophomenidae and Pentameracea of the 

 Protremata. It is rarely possible to visit a fossiliferous 

 horizon in Proterozoic strata without discovery of many 

 species that would have been called Orthis by early 

 Palaeontologists ; while Pentamerids are important rock- 

 formers in the Silurian. The Trilobites are the most 

 striking, and not the least characteristic, fossils of the 

 Lower Palaeozoic. Hypoparia and Opisthoparia abound 

 in the Cambrian, while the Proparia give them full 

 support in the Ordovician. Although Trilobites range 

 up to the Permian, post-Silurian types are usually 

 scarce in occurrence and gerontic in character. The 

 Ordovician is the period of their acme, twenty-two 

 families (out of a total of twenty-eight) being there 

 represented. Trilobites are used as zonal indices in 

 the Cambrian (in the absence of Graptolites), but the 

 world-wide range of many Ordovician and Silurian 

 genera gives those later types great stratigraphical 

 value, slightly discounted by tendencies to specific 

 longevity. 



