492 FOSSIL BRACHIOPODA chap. 



Since all Bracliiopods are inhabitants of the sea, the geologist 

 at once recognises as a marine deposit any bed which contains 

 their remains. Under favourable conditions they swarmed in the 

 seas of Palaeozoic and Mesozoic times. Beds of limestone are fre- 

 quently almost entirely com^^osed of their shells, as, for instance, 

 some of the Devonian limestones of Bohemia. Often they give 

 the facies to the fauna and outnumber in species and individuals 

 all the other organisms of the period. The Ungulite Sandstone 

 (Cambrian) of Russia and the Productus Limestone of the Salt 

 Range in India of Carboniferous and Permian age are well- 

 known examples. 



Many species seem to have been gregarious in habit; thus 

 Productus giganteus of the Carboniferous Limestone may gen- 

 erally be found in crowded masses, as in some localities in 

 Yorkshire. 



The fact that certain species of Brachiopods characterise 

 definite stratigraphical horizons or " zones " gives them occasion- 

 ally an importance equal to that of Graptolites ; for instance, the 

 Ecardinate species Trematis corona marks a set of beds in the 

 Ordovician, and the isolated Stringocephalus Burtini is restricted 

 to the upper part of the Middle Devonian, giving to the lime- 

 stone on that horizon its distinctive name. It is noteworthy also 

 how certain species affect a sandy and others a calcareous sea- 

 bottom, so that beds of the same age show differences in their 

 Brachiopod fauna owing to a dissimilar lithological composition. 



While few of the recent Brachiopods reach a large size, some 

 of the extinct species measure several inches in breadth, but the 

 great Productus giganteus attained the width of even a foot. 



The bright colours of the shells of the living animals are 

 not generally preserved amongst the fossil species from the older 

 rocks ; yet in a Carboniferous Terehratula we can even now 

 detect the purple bands in some specimens, and a Cretaceous 

 Rhynchonella similarly exhibits its original colour. 



The Brachiopoda are evidently a group in its decline, as the 

 geological record shows ; but they date back from the earliest 

 known fossiliferous rocks, in which the Ecardinate division 

 is alone represented. As we ascend through the stratigraphical 

 series the number and variety of genera and species belonging to 



Brit. Foss. Brack. (Palaeont. Soc, 1851-1884). Waagen, Salt Bange Fossils 

 (^Mem. Geol. Surv. India, 1879-1885). 



