258 PALAEOZOIC TIME UPPER SILURIAN. 



amounted in Pennsylvania to at least 500 feet in the Oneida 

 epoch, 1500 feet in the Medina, over 2000 feet in the Clinton, 1500 

 feet in the Niagara and Salina, and 500 in the Lower Helderberg, — 

 in all 6000 feet. In the Salina period, the subsiding area stretched 

 up into New York west of its centre; for it was there that the 

 Salina beds were formed to a thickness of 1000 feet. 



Life. — (a.) General features. — The range of animal life was in 

 its grander divisions the same as in the later part of the Lower 

 Silurian. The highest species continued to be the Cephalopods, — 

 the first among Mollusks, and that group among Invertebrates 

 that more than any other embraced characters of the Vertebrates ; 

 for example, perfect organs of sight, hearing, and touch, great size 

 and strength, and powerful arms for prehension. The Crustaceans 

 belong to a higher type, — the Articulate, — but only to the lower 

 division of that type. The Vertebrates are yet unknown among 

 American Silurian fossils. 



While the grand types remained the same, there were changes 

 through the disappearance of many genera and families and the 

 introduction of others. 



(b.) Radiates. — The group of Graptolites, which passed its climax 

 in the Lower Silurian, had its last species in the Clinton epoch of 

 the Upper Silurian. Crinideans were brought out in many new 

 genera and an increasing number of species. Corals became much 

 more varied ; the Chain-corals (Hah/sites) were common, but passed 

 away with the Upper Silurian ; the Favosites and the Cyathophyl- 

 loids also increase in abundance, and abound still later in the 

 Devonian. 



(c.) Mollusks. — Mollusks — the dominant type of the seas — are 

 most abundantly represented by Brachiopods. Among them, the 

 genera Spirifer, Athyris, Chonetes, and others, were added to IAn- 

 gula, Orthis, Leptcena, Rhynchonella, Atrypa, etc. of the Lower Silu- 

 rian ; at the same time, Orthis had lost its pre-eminence, and was 

 of few species. The Lower Silurian Brachiopods have no bony 

 arm-supports internally, excepting the very short ones in Rhyncho- 

 nella. In both Spirifer and Atrypa these supports were long and 

 rolled spirally. The genus Spirifer commences with narrow spe- 

 cies, little broader than high, but in the later part of the Upper 

 Silurian they are already much wider (fig. 429), though not as 

 extravagantly so as in many of the species in the Devonian and 

 Carboniferous ages. 



The Conchifers and Gasteropods are few compared with the 

 Brachiopods ; and in both groups the species are mostly siphon- 

 less ; that is, the Gasteropods have the aperture without a beak, 



