182 



PALAEOZOIC TIME — LOWER SILURIAN. 



The most abundant fossils in the Potsdam beds are the shells of 

 the Brachiopod genus Lingula (figs. 237-239), and Trilobites. 



The Lingulse are so numerous in some places as to give their 

 name to the rock : thus, there are the Lingula grits and Lingula flags, 



2. Spirifer Family. — The genus Spirifer includes the common species, having 

 usually a long hinge-line and distinct cardinal area (figs. 211, 212). In Athyris 

 (fig. 213) the hinge-line is much shorter, the hinge-area small or none, the beak 

 contracted and having a small round aperture. This genus is like Terebratula in 

 its narrow form, and beak without cardinal area, but has the spires of the Spiri- 



Figs. 216-227. 



Fig. 216, Rhynchonella psittacea, showing the spiral arms of the animal ; 217, id. dorsal 

 valve; 218, id. ventral; 219, Strophomena planumbona; 220, id. dorsal valve; 221, id. 

 ventral; 222, Leptama transversalis ; 223, id. dorsal valve; 224, id. ventral; 225, Orthis 

 striatula ; 226, id. dorsal valve ; 227, id. ventral. 



fers. Atrypa (figs. 214, 215) has the spires differently arranged, as in the figure; 

 the form narrows to the beak, where there is no hinge-area or only a small one. 

 Uncites has the beak extravagantly prolonged, and a large opening beneath it. 

 Cyrtia has nearly the same extravagant prolongation of the beak, but with a 

 large hinge-area, and a very small opening left at the top of the pseudo-deltidium. 



