496 



FOSSIL GENERA OF BRACHIOPODA. 



Fig. 118. Spirifera. 



several genera are characterised by the internal shelly plates, loops, 

 and spirals (see fig. 118) which support the breathing organs. Most 

 of the genera are extinct and confined to the Primary rocks, though 



several primary genera range up to the 

 lower Oolites. We first enumerate 

 some of the extinct genera. Spiri- 

 fera .and. Atrypa range through tlie 

 Primary to the Trias, Orthis and 

 Strophomena only range up to the 

 Carboniferous, Obolus and Siphonotrda 

 are Cambrian and Silurian, Untites 

 is Devonian, and Stringocephalus, 

 Merista, and Pentamerus are Silurian 

 and Devonian ; Chonetes ranges from Cambrian to Permian, and Pro- 

 ducta and Stroplialosia from Devonian to Permian. Several genera 

 extend up to the Trias; among them are Cyrtia, ftetzia, 

 which both date from the Silurian ; and Davidsonia, which 

 dates from the Devonian. Other genera range up to the 

 Lias ; among them are Leptcena and Athyris. Koninckia is 

 confined to the Trias ; Suessia is confined to the Upper 

 Lias. Spiriferina extends from the Devonian to the Lower 

 Oolite, Zellania extends from the Lias to the Great Oolite. 

 F Sa"J'~~ Ki n ff ena > Magas, Trigonosemus, and Lyra are Cretaceous. 



Of genera which date from the older primary rocks and 

 still survive, the most important are Lingula, Crania, Discina, and 

 Rhynchonella. Terebratula survives from the Devonian, Waldheimia 

 from the Trias. Thecidium first appears in the Carboniferous, Tere- 

 bratella in the Lias, Argiope in the Inferior Oolite ; the living Tere- 

 hratulma is found in the Oxfordian rocks. 1 



Fig. 120. Rhynchonella. 



Fig. 121. Terebratula. 



Fig. 122. Terebratula. 



At the present day Brachiopods live in all depths of water and in 

 all seas. 



Lamellibranchiata. The unsymmetrical bivalve shells vary 

 greatly in form. Occasionally some species of Pecten and a few other 

 genera have the right and left sides of the shell equal and similar, 

 and although the right and left valves of the shell are commonly 

 equal, yet in many genera, such as Ostrea, Corbula, and the common 

 scallop, the valves are unequal. As among the other large groups of 



1 See Davidson, " Fossil Brachiopoda " (Palaeontographical Society) ; and 

 Zittel, "Handbuch d. Palseontologie. " 



