506 FOSSIL BRACHIOPODA chap. 



Family : Tercbratididae 

 Shell structure punctate. 



Arms supported by a calcareous loop, usually bent back on itself. 

 (For muscular impressions see p. 502, Figs. 328, 329.) 

 Beak of ventral valve perforated by foramen, furnished with deltidium. 

 Range. — Devonian to Recent ; maximum development in Mesozoic times. 

 Principal Genera. — Terebrahda, Terebrahdina, TValdheiviia, Terebratella, 

 Kingena, Macjas, Centronella. 



Famil}' : ArgiojAdae. 



Large foramen for passage of pedicle. Marginal septa present in both 

 valves. Calcareous brachial loop follows margin of .shell and is more or less 

 fused with the septa. Shell structure punctate. 



Range. — Jurassic to Recent. 



Principal Genera.- — Argiope, Cistella. 



Family : Stringocepludidae 



Shell subcircular, punctate. Cardinal process highly developed, bifid. 

 Brachial apparatus composed of two calcareous free lamellae, prolonged at 

 first downwards, then bent back, upwards and outwards to run parallel to 

 margin of shell and to unite in front, thus constituting a wide loop. 



Range. — Silurian and Devonian. 



Sole Genus. — Stringocephalv.'. 



Family : Thecidiid'ie 



Shell usually fixed by beak of ventral valve, plano-convex. Sub-cardinal 

 apophysis in ventral valve for attachment of occlusors. Marginal sejita in 

 dorsal valve. Calcareous brachial loop more or less fused with shell, and 

 with calcareous spicules of mantle. Shell structure : inner layer fibrous, 

 outer layer tubulated. 



Range. — Carboniferous to Recent. 



Principal Genera. — Thecidium, Uldliamina. 



Stratigeaphical Distpjbution of Brachiopoda 



It is remarkable that some of the earliest types of Brachiopoda 

 exist generically unchanged at the present day. Such are Lingula, 

 ranging from the Cambrian ; Disci7ia, and Crania, ranging from 

 the Ordovician ; and amongst the hinged forms Terehratula from 

 the Devonian, and Rliynchonella from the Ordovician. 



In the lowest Cambrian (Olenellus beds) the most important 

 genera are Linnarssonia and Kutorgina. The hinged forms 

 appear in the Cambrian, being represented by Orthis ; but the 

 majority in this formation belong to the Ecardines. Lingula, 

 Lingulella, and Oholella are characteristic. 



In the Ordovician many new genera of the Testicardines 



