CLASS II BRACHIOPODA 397 



Thick-shelled, often gibbous Rhynchomllae with prominent, thick, concave 

 deltidial plates. Dorsal valve with a thick median septum, upon which rests 



B 



A 



C 





Fio. 590. 

 Rhynchotrema capax (Conrad). Ordovician ; Wisconsin. A, Interior of ventral valve sliowing concave 

 deltidial plates {dp) and teeth (0- B, Dorsal view. C, Dorsal interior showing cardinal process (j), crural 

 processes (c) and dorsal sockets (h). (After Hall and Clarke.) 



a linear cardinal process. Crural plates very broad and stout. Ordovician ; 

 North America. 



Stegerhynchus Foerste. Exterior as in Camarotoechia^ but with a thin 

 vertical cardinal process as in Rhynchotrema. Silurian ; North America and 

 Europe. 



Rhynchotreta Hall. Trihedral Rhynchonellae with the ventral beak acu- 

 minate, produced and truncate. Pedicle foramen apical, the delthyrium 

 being completely closed by the deltidial plates. Dental lamellae and car- 

 dinal process present. The prominent dorsal median septum separates 

 posteriorly, each brauch supporting one process of the divided hinge-plate. 

 Silurian ; North America and Europe. 



Eatonia Hall. Rhynchonellae with large, flabellate, deeply excavated 

 muscular scars in the ventral valve. No dental lamellae. Cardinal process 

 large, resting upon a short median septum, and bifurcate at its summit. 

 Devonian ; North America. 



Uncinulus Bayle. Like fVilsonia, but with the hinge-plate undivided, and 

 with a well-developed cardinal process. Devonian ; North America and 

 Europe. 



(?) Cyclorhina Hall and Clarke. May be of the Athyridae rather than of 

 the Rhynchonellidae. Devonian ; North America. 



Subfamily B. Rhynchonellinab Gill. 



Rhynchonellids without a cardinal process. Silurian to Recent. 



Camarotoechia Hall and Clarke (Fig. 591). Sharply plicate Rhynchonellae 

 with the dorsal median 



septum bearing pos- >**. ^ B ,hD ^' 



teriorly a short crural 



cavity. Cardinal process ^^/jl^^^^ <^|^|^4^- ,^||||||Q^J^v 

 absent ; dental lamellae 

 present. Silurian to 

 Lower Carboniferous ; p^^. ^^^^ 



North America and CamarotoecUa congregata {Conv&A). Devonian ; New York. ^.Dorsal 



Für one aspect. B, C, Dorsal interiors showing hinge-plate (hp), rostral Chamber 



o-iui upc. ^^^^ crural processes and dental sockets (h). (After Hall and Clarke.) 



Plethorhyncha Hall 

 and Clarke. Large, ponderous Rhynchonellae, with almost no dental lamellae 

 and 110 crural cavity. Lower Devonian ; North America and Europe. 



