272 



PALEOZOIC TIME. 



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 to the south, there were still some 

 large species of corals and Crinoids ; but the 

 predominant fossils were Brachiopods and 

 Lamellibranchs — species that live on muddy 

 bottoms. There were many broad-winged 

 Spirifers, among which the Sp. mucronatus 

 (Fig. 539) was very common. The limestone 

 layers mark an occasional change to clearer 

 waters, when crinoids and corals had a chance 



Heliophyllum Halli. %q ^^ 



Brachiopods continued to be the most common of fossils. 

 536 to 543 represent the most common kinds. 



Figs. 536-543. 



Figures* 



Brachiopods. — Fig. 536, Afcrypa aspera; 537. A. reticularis; 538, Tropidoleptus carinatus ; 539, 

 Spirifer mucronatus ; 540, Athyris spiriferoides ; 541, Spirifer (Martinia) umbonatus ; 542, Cho- 

 netes setigera ; 543, Productus subalatus." 



Lamellibranchs are more numerous than in former eras. The fol- 

 lowing figures, 544 to 547, show some of the characteristic species. 



With this period commenced the genus Goniatites (Fig. 548) — 

 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. 548 a. The Goniatites 

 became more and more complex in the flexures of the septa during 

 the following periods of the Paleozoic, and afterward were replaced 

 by the Ceratites and Ammonites, to which they are closely related. 



