RHYNCHONELLID^. Sit 



Yalve is so much developed as to give the whole shell the external 

 appearance of an Orthis. 



Camerella, Billings, 1859. 



;Si/n.— Triplesia, Hall, 1859. 



Distr. — 9 sp. Lower Silurian ; North America. G. Volhorthi, 

 Billings. 



Yentral valve with a small triangular chamber beneath the 

 beak, supported by a short mesial septum as in Pentamerus. 

 Dorsal valve with a single mesial septum and two short lamellae 

 for the support of the oral appendages, as in Rh^aichonella. 

 Surface smooth or obscurely plicated. 



EiCHWALDiA, Billings, 1858. 



Etym. — Dedicated to Professor Eichwald, the celebrated 

 Russian paleeontologist. 



Distr. — 3 sp. Silurian ; Canada, England. E. subtrigonalis, 

 Billings. 



Shell with the ventral valve perforated on the umbo for the 

 passage of a peduncle; the place of the foramen beneath the 

 beak being occupied by an imperforate concave plate ; the 

 interior of each valve divided by a medio-longitudinal ridge, 

 that of the dorsal valve very prominent; hinge and teeth sockets 

 wanting. 



The internal structure of the ventral valve somewhat resem- 

 bles that of Pentamerus or Camarophoria. 



Camarophoria, King, 1844. 



Distr. — Fossil, 9 sp. Carb. — Permian (Magnesian limestone) ; 

 Germany and England, N. America. G. Schlotheimi, Busch 

 (cxxxvi, 82). G. crumeiia, Martin (cxxxvi, 83, 84). 



Ventral valve with converging dental plates supported on a 

 low septal ridge ; dorsal valve with a prominent septum sup- 

 porting a spoon-shaped central process ; oral lamellse long and 

 slender. Foramen angular, cardinal process distinct. 



Pentamerus, Sowerby, 1813. 



Etym. — Pentameres, five-partite. 



Distr. — Possil, 52 sp. Upper Silurian — Devonian ; Arctic 

 America, United States, Europe. P. Knightii, Sby. (cxxxvi, 

 85-87). 



Shell impunctate, ovate, ventricose, with a large incurved 

 beak ; valves usually plaited ; foramen angular ; no area or del- 

 tidium ; dental plates converging, trough-like, supported on a 

 prominent septum ; dorsal valve with two contiguous longitudinal 

 sept:i opposed to the plates of the other valve. 



Oral lamellae have been detected by Mr. Salter in P. liratus ; 



