284 PALEOZOIC TIME — DEVONIAN AGE. 



Genesee slate there is a return to the fineness of the Marcellus, 

 and also in part to some of the same species of shells. 



The preceding period had abounded in corals, and hence in lime- 

 stones ; in the Hamilton, when the condition was unfavorable for 

 coral reefs over New York and south, there were still some fine 

 species of corals and Crinoids, but the predominant fossils were 

 Brachiopods and Conchifers, — species that live on muddy bottoms. 

 There were many broad-winged Spirifers, among which the Sp. mu- 

 cronatus (fig. 489) was very common. The limestone layers mark an 

 occasional change to clearer waters, when Crinoids and corals had 

 a chance to flourish. 



With this period commence the earliest of Goniatites (fig. 498), — 

 a group of Cephalopods with Nautilus-like shells, but differing 

 from Nautilus in having the siphuncle dorsal, and the septa with 

 one or more flexures at the margin ; in case of one flexure or more, 

 there is always one on the dorsal margin, as in fig. 498 a. The 

 Goniatites became more and more complex in the flexures of the 

 septa during the following periods of the Palaeozoic, and after- 

 wards were replaced by the Ceratites and Ammonites, to which 

 they are closely related. 



Characteristic Species. 



1. Radiates. — Fig. 485, the Coral HelipphyUum Haiti, common in the 

 Hamilton at Moscow, York, and elsewhere, and found also in England. The 

 Encrinal limestone is made up of fragments of 



crinoidal columns. * 1 £- 485. 



2. Mollusks. — (a.) Brachiopods.— Fig. 486, 

 Atrypa aspera, also European ; 487, A. reticularis, 

 regarded as the same species as that of the Cornife- 

 rous period, but usually much larger and fuller,being 

 sometimes nearly two inches broad ; 488, Tropidolep- 

 ius carinatus, in New York, Illinois, Iowa, Europe ; 

 489, Spirifer mucronatus, very common ; 490, Athy- 

 ris spiriferoides {Atrypa concentrica of Conrad), 

 — it has the spire internally of a Spirifer ; 491, 

 Spirifer [Martinia) umbonatus, also European; 

 492, Chonetes setigera, found in both the Marcellus 



•and Genesee shales; 493, Productus subalatus, 

 Rock Island, 111. A shell closely like the S. um- 

 bonatus, but higher, occurs in Iowa and Illinois, and is named Cyrtia umbonata 

 by Hall. Spirifer granuliferus is a large Hamilton species, having a granu- 

 lated surface. 



(b.) Conchifers. — The species are often of large size, but none yet described 

 have a sinus in the pallial impression. Fig. 494, Orthonota undulata Con.; 

 495, Avicula Flabella ; 496, Grammysia Hamiltonensis, — also European, in the 

 Eifel; 497, Microdon bellistriatus Con. 



