420 MOLLUSCOIDEA phylum v 



Three of the four Orders into which the class is divided are represented in 

 the lowest Cambrian, or Olenellus zone, indicating that Brachiopods had their 

 origin in pre-Cambrian times. In the Lower and Middle Cambrian, the 

 Atremata and Neotremata predominate ; and although the Protremata are known 

 in the Lower Cambrian by very typical species, it is not until the Upper 

 Cambrian that the order becomes conspicuous. They are particularly char- 

 acteristic of the Paleozoic. The Telotremata do not appear until the Middle 

 Ordovician and since Silurian times have always been prolific, but are 

 especially characteristic since early Mesozoic times. 



In the Lower Cambrian (Olenellus beds), 22 genera of Brachiopods are 

 represented, occurring both in North America and in Europe. A marked 

 increase is apparent in the Middle Cambrian, for here Walcott records 37 

 genera, and in the Ordovician and Silurian, where the climax of their diversity 

 is reached, upwards of 3000 species are known. These are distributed chiefly 

 in North America and in Europe (Great Britain, Scandinavia, Bohemia, 

 Russia and Portugal) ; but numerous forms are also found in South America, 

 Australia, China and eastern Siberia. 



In the Devonian, Brachiopods are scarcely less plentiful than in the Silurian, 

 although a considerable number of genera, especially those belonging to the 

 Atremata and Neotremata, have now disappeared. The most noted European 

 localities where Brachiopods abound are the Eifel, Rhineland, Westphalia, the 

 Hartz, Belgium, Devonshire, Boulogne-sur-Mer, Cabrieres in the Cevennes, 

 the Asturias and the Urals. North America also yields great quantities of 

 Devonian Brachiopods. 



The Carboniferous of North America and its equivalent horizons in Europe 

 and Eastern Asia, together with the Permian of the Mediterranean countries, 

 India and Armenia, are very rieh in Brachiopod remains, especially those 

 belonging to the Produdidae, Strophomenidae, Spiriferidae and Rhynclionellidae. 



In the North European Permian, the number of species of Brachiopods is 

 reduced to about 30, but in the Salt Range of India far greater numbers 

 occur. In the Alpine Trias, the Terehratulidae, Rhynchonellidae and Koninck- 

 ininae attain a great development. 



The Jurassic and Cretaceous Brachiopods belong almost exclusively to 

 the Terehratulidae, Rhynchonellidae and Thecidiidae ; the first two families in 

 particular being represented by an astonishing number of species. The 

 Spiriferacea become extinct in the Lias. 



With the beginning of the Cenozoic era, Brachiopods are no longer a con- 

 spicuous group of fossils. The species occurring in the Tertiary are almost 

 without exception generically identical with those now living, and scarcely 

 exceed them in number. On this account they are devoid of practical interest 

 or importance to the geologist. 



