502 



LECTURE XX. 



closely adherent mantle of two widely separated lobes. Oral tentacles 

 two, long, fringed, usually more or less spirally disposed. 



Family TerehratuUdce. 



Ventral valve with a prominent notched or perforated beak, and 

 two curved hinge-teeth : dorsal valve with a depressed umbo, a 

 cardinal process between the dental sockets, and an internal calca- 

 reous, usually, looped brachial appendage. Animal attached by a 

 short pedicle or by a ventral valve. 



Genera Terebratula, TerehratuUna, Waldheimia, Terebratella, 

 Magas, Bouchardia, Morrisia, Kraussia, Argiope, Thecidium, 

 Stringocephalus. 



Family Spiriferidce. 



Ventral valve with prominent hinge-teeth, and slightly produced 

 and notched beak. Dorsal valve with a small cardinal process, a 

 divided hinge-plate, and a pair of internal slender calcareous brachial 

 appendages disposed in numerous close spiral coils, nearly filling the 

 shelL 



Genera Spirifer, Athyris^ Retzia, Uncites. 



Family RhynclioneUidoe, 



Ventral valve with a prominent notched beak ; giving passage to a 

 peduncle : hinge-teeth supported by dental plates. Dorsal valve 

 with a deeply divided hinge-plate; and two short, rarely spiral, 

 internal brachial processes. 



Genera Rhynchonclla, Pentamcrus, Atrypa. 



Family Orthidce. 



Both valves with short subequal beaks ; a straight and wide hinge- 

 plate, notched in the middle. Ventral va^ve with prominent teeth: 

 dorsal valve with a tooth-like cardinal process, between two curved 

 brachial processes. 



Genera Orthis, Strophonema, LeptcEna, Koninckia, Davidsonia, 

 Calceola. 



Family Productidce. 



Ventral valve convex, with a slightly notched hinge line : dorsal 

 valve concave, with a prominent cardinal process and subcentral 

 short brachial processes. 



Genera Producta., Strophalosia, Chonetes. 



