POTSDAM PERIOD. 



183 



among the Primordial strata ; and the same is true of a closely- 

 related shell of the genus Obolus (fig. 236). Besides Brachiopods 

 of the Lingula family, there are others of the Orthis and Ehyncho- 

 nella families even in the Potsdam epoch. 



Koninckina is an imperfectly determined genus, resembling Productua in form, 

 but differing internally. 



Among other genera and subgenera of this family may be mentioned Cyrtina, 

 Retzia, Merista, Nucleospira, Trematospira, Rhynchosjiira, Charionella, etc. 



3. Rhynchonella Family. — Tbe genus Rhynchonella (figs. 216-218) contains 

 plump-ovoid or subtrigonal shells, usually narrower than high, and narrowing to 

 the beak, having usually a foramen and no hinge-area; generally a U-shaped 

 flexure in the anterior margin of the shell. Pentamerus has a much fuller and 

 more incurved beak, and no area or deltidium, though there is a triangular 



228-236. 



Fig. 228, Productus aculeatus, dorsal view; 229, Productus semireticulatus, ventral view; 

 229 a, section of Productus, showing the curvature of the valves; 230, Chonetes lata, 

 opposite views; 231, Calceola sandalina; 232, Crania antiqua; 233, Discina lamellosa, side- 

 view; 234, id. showing foramen; 235a, b, Siphonotreta unguiculata, opposite views; 236, 

 Obolus Apollinis. 



opening at the middle of the hinge, which usually becomes closed in adult shells 

 by the incurving of the beak. Camarojjhoria is a rare genus of the Carboni- 

 ferous and Permian. Porambonites, a very plump shell of the Lower Silurian, 

 near Rhynchonella. Comerella of Billings is another genus of this family, 

 found in the Lower Silurian. Leptocoe.Ua and Eatonia probably belong to this 

 family. 



4. Orthis Family. — In the genus Orthis (figures 225-227) the species are 

 usually rather thin; often orbicular, at times a little wider than high; both 



