XVIII STRATIGRAPHICAL DISTRIBUTION 507 



In the Ordovician many new genera of the Testicar dines 

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

 RhyncJionella^ Clitarnbomtes, 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^ Oonchidium^ Stricklan- 

 dia^ Chonetes^ Cyrtia, etc.); and the Trimerellidae are especially 

 characteristic of the Wenlock. 



With the commencement of Devonian times many species 

 and genera become extinct, but new forms come in QTerehratula^ 

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

 fined to this formation ( Uncites, String ocephalus) . The Carbonife- 

 rous is marked by the maximum development of Productiis and 

 Spirifera; 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 StenocMsma; 

 but Aulosteges 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 QSpiriferina^ Athyris)^ and the genus Kojiinckina is 

 restricted to this formation. 



The enormous development of species of the Terehratulidae 

 and RJiynchonellidae is the most noticeable feature in Jurassic 

 times; and a few ancient types linger on into the Lias QSpiri- 

 ferina^ Suessia, a sub-genus of Spirifera'); Koninckella here occurs. 



The Cretaceous Brachiopoda are closely allied to the Juras- 

 sic; Magas and Lyra are peculiar to the period, and the Tere- 

 hratulidae and Rhynchonellidae 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 : — 



