5o6 FOSSIL BRACHIOPODA 



CHAP. 



Family. Terehratulidae 



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 deltidinm. 



Range. — Devonian to Recent ; maximum development in Mesozoic 

 times. 



Principal Genera. — Tei^ehratula, TerebratuUna, Waldheimia, Terebra- 

 tella, Klngeiia, Magas, Centronella. 



Family. Argiopidae 



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 jDunctate. 



Range. — Jurassic to Recent. 



Principal Genera. — Argiope, Cistella. 



Family. Stringocephalldae 



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 jDarallel to 

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



Range. — Silurian and Devonian. 



Sole Genus. — Stringocephalus. 



Family. Thecidiidae 



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

 apophysis in ventral valve for attachment of occlusors. Marginal septa 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, Oldhamina. 



Stratigraphical Distribution of Brachiopoda 



It is remarkable that some of the earliest types of Brachio- 

 poda exist generically unchanged at the present day. Such are 

 Lingula^vdug'mg from the Cambrian; Discina and Crania^ ranging 

 from the Ordovician ; and amono-st the hinofed forms Terehratula 

 from the Devonian, and Rhynclionella from the Ordovician. 



In the lowest Cambrian (Olenellus beds) the most important 

 genera are Liyinar^Honia and Kutorgi)ta. The hinged forms 

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

 majority in this formation belong to the Ecardines. Lingula^ 

 Lingulella^ and Oholella are characteristic. 



