XVIII STRATIGRAPHICAL DISTRIBUTION 507 



In the Ordovician many new genera of the Testicardines 

 make their appearance, such as Strophomena^ Leptaena^ Atrypa^ 

 Rhi/nchonella^ Clitambomtes^ etc., but the extraordinary abun- 

 dance and variety of Orthis is most remarkable. The Ecardines 

 are reinforced by such forms as Trematis and Siphonotreta. It 

 is, however, in the Silurian that the Testicardinate Brachiopoda 

 attain their maximum, for in addition to a great development 

 of species amongst the older forms, a host of new genera for the 

 first time occur here (^Spirifera^ Athyris^ Concluclium^ Stricklayi- 

 dia, Chonetes, Cyrtia^ etc.); and the T rimer ellidae are especially 

 characteristic of the Wenlock. 



With the commencement of Devonian times many species 

 and genera become extinct, but new forms come in (^Terebratula, 

 Orthothetes^ Productus^ etc.), and some genera are wholly con- 

 fined to this formation ( Uncltes^ Stringoeephalus) . The Carbonife- 

 rous is marked by the maximum development of Productus and 

 Spirlfera; Orthothetes, Stenochisma^ and Athyris are also abun- 

 dant, but there is a considerable extinction of the older genera 

 and species, and a great diminution in the number of individuals 

 and species of those that persist. 



A further reduction occurs in the Permian, where the most 

 important genera are Productus^ Strophalosia^ and Stenochisma; 

 but Auloateges is a new form peculiar to this period. In the 

 Trias a new era commences; the principal families and genera 

 of the older rocks disappear entirely; a few spire-bearing genera 

 persist (Spiriferlna^ Athyris^, and the genus Koninckina is 

 restricted to this formation. 



The enormous development of species of the TerehratuUdae 

 and RhyncJionellidae is the most noticeable feature in Jurassic 

 times; and a few ancient types linger on into the Lias (^Spiri- 

 ferina^ Suessia, a sub-genus of Spirifera)\ KoninckeUa here occurs. 



The Cretaceous Brachiopoda are closely allied to the Juras- 

 sic; Magas and Lyra ai'e peculiar to the period, and the Tere- 

 hratuUdae and RhynchonelUdae are very abundant, together with 

 the Ecardinate genus Crania. 



With the commencement of Tertiary times the Brachiopoda 

 have lost their geological importance, and have dwindled down 

 into an insignificant proportion of the whole Invertebrate fauna. 



The distribution of the Brachiopoda in past time is shown 

 in the following table : — 



