NIAGARA PERIOD — NIAGARA EPOCH. 237 



(b.) Brachiopods. — There are species of Lingula, Orthis, Leptaena, Rhyncho- 

 nella, Spirifer, and also of the new genera for America, Chonetes and Pentamerus. 

 Fig. 371, Pentamerus oblongus ; some specimens are more than twice the size of 

 this figure, and very thick ; it is abundant in New York State and the West, and 

 occurs also in Britain ,• figs. 372, 373 show casts of the interior, — 372 a dorsal 

 view, and 373 a ventral. Figs. 374, 375, Atrypa reticularis, or a related species; 

 the A. reticularis is reputed to extend through the Niagara period into the 

 Hamilton of the Devonian ; but more than one species are probably here in- 

 cluded; this also is a foreign species: it is one of the few species of true Atrypa; 

 the interior of the shell is shown in fig. 215. Fig. 376, Athyris f congesta ; fig. 

 377, same, different view, — it has a spire within, extending downward and out- 

 ward ; fig. 378, Chonetes cornuta Koninck. 



(c.) Conchifers. — Fig. 379, Avicula rhomboidea H. 



(d.) Gasteropoda. — Fig. 380, Cyclonema cancellata H. Bucania trilobata of the 

 Medina also occurs here, besides other Univalves. 



(e.) Cephalopoda. — There are a few Orthocerata. 



3. Articulates. — Remains of Trilobites of the genus Homalonotus, and 

 of the same species figured under the Niagara epoch. Tracks or scratches occur 

 which have been referred with good reason to Crustaceans, besides others like 

 fig. 382, that are attributed to "Worms. 



Among the Clinton species are the following from the Lower Silurian : — Orthis 

 Lynx, Leptsena sericea, Bellerophon bilobatus. The following are known in 

 Europe : — Orthis Lynx, Chonetes cornuta, Atrypa reticularis, A. hemispherica, Spi- 

 rifer radiatus, Pentamerus oblongus. 



NIAGARA EPOCH (5 d). 



I. Rocks : kinds and distribution. 



The Niagara and Clinton epochs had several points of similarity. 

 (1.) The formations thin out to the eastward in New York. ,(2.) 

 The rocks are shales and limestone, but the latter increases in pro- 

 portion in central and western New York, and becomes the 

 prevailing material of the formation through the interior of the 

 Mississippi basin, while along the Appalachians in Pennsylvania 

 and farther south the rocks are almost solely shales. (3.) A con- 

 siderable number of the Clinton fossils reach up into the Niagara 

 formation. 



On the contrary — (1.) The Niagara beds spread more widely 

 over the continent, — occurring through a large part of the Interior 

 Continental basin, from New York to beyond the Mississippi, and 

 from Michigan to Tennessee, as well as along the Appalachian region 

 in Pennsylvania and Virginia to Alabama ; also at various places in 

 the Arctic, and in other parts of British America, as on the borders 

 of Hudson Bay. (2.) The great majority of the species are not 

 found in the Clinton beds, and the species are more generally clear- 



