CLASS II 



BRACHIOPODA 



397 



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

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



B 



Fio. 500. 



Rhynclwtrctna capax (Conrad). Ordovician ; Wisconsin. A, Interioi' of ventral \'alve sliowing concave 

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

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



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

 North America. 



StegerhyncJms Foerste, Exterior as in Camarotoechia, but with a thin 

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

 Europe. 



Ehynchotreta Hall. Trihedral Bhynchonellae 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 branch supporting one process of the divided hinge-plate. 

 Silurian ; North America and Europe. 



Eatonia Hall. Bhynchonellae 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 JFilsonia, but with the hinge-plate undivided, and 

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

 Europe. 



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

 the Rhynchonellidae. Devonian ; North America. 



Subfamily B. Rhynchonellinae Gill. 



Rhynchonellids without a cardinal process. Silurian to Recent. 

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



A 



with the dorsal median 

 septum bearing pos 



a short crural 



Cardinal process 



dental lamellae 



Silurian to 



Carboniferous ; 



America and 





teriorly 



cavity. 



absent ; 



present. 



Lower 



North 



Europe. 



Plethorhyncha Hall 

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

 and no crural cavity. Lower Devonian ; North America and Europe. 



Pio. 591. 



Camarotoechia congregata (Conrad). Devonian; New York. A, Dorsal 

 aspect. B, C, Dorsal interiors showing hinge-plate (hp), rostral chamber 

 (d), crural processes and dental sockets (h). (After Hall and Clarke.) 



