186 BRACHIOPODA 



southern regions. A few species, as for example Terebra- 

 tulina caput-serpentis, have a very wide geographical 

 distribution, extending from polar to tropical regions, and 

 also have a great range in depth, the form mentioned 

 being found from the shore-line down to 1180 fathoms. 



The species found in deep water have generally a 

 much wider geographical range than those confined to 

 shallow water; and the polar or boreal species have a 

 wider range than the tropical, since, in lower latitudes, 

 they can find a suitable temperature at greater depths. 



Brachiopods are very abundant as fossils, especially in 

 the Palaeozoic and Mesozoic formations. The earliest 

 forms occur in the Lower Cambrian (Olenellus Beds), 

 where no less than thirteen genera are represented, of 

 which Lingulella, Iphidea, Kutorgina, and Obolella may be 

 mentioned. The majority of the species found in the 

 Cambrian belong to the Inarticulata, but the two Orders 

 of the Articulata are also represented. In the Ordovician 

 System the Brachiopods (especially the Articulata) are 

 much more numerous than in the Cambrian, and they attain 

 their maximum in the Silurian ; their decline begins in 

 the Devonian ; in the Mesozoic it is especially marked by 

 the reduction in the number of genera represented. The 

 chief genera met with in the different systems are : — 



Cambrian. Lingula, Lingulella, Kutorgina, Obolella, ' Discina,' 

 Orthis. 



Ordovician. Lingula, Siphonotreta, Orthis, Strophomena, Lep- 

 tama. 



Silurian. Lingula, Orbicidoidea, Airy-pa, Orthis, Meristina, 

 Spirifer, Cyrtia, ' Rhynchonella ' {Rhynchotreta, Camarotoechia, Wil- 

 sonia, etc.), Pentamerus, Stricklandia, Leptcena, Strophonella, 

 Strophomena, Chonetes. 



Devonian. Uncites, String ocephalus, Athyris, A try pa, Orthis, 



