586 HISTORICAL GEOLOGY. 



which, covered with spines, is represented in Fig. 873; also species of 

 Bellerophon, Euo7nphalus, Pleurotomaria, Murchisonia, Lower Silurian genera 

 that continue through the Devonian, into, and part of them beyond, the Car- 

 boniferous. Among Cephalopods there are species of Orthoceras, Gompho- 

 ceras, Cyrtoceras, and the last known of Trochoceras, a Silurian type ; also the 

 first known of American Goniatites — G. mithrax H. (Fig. 874), and a variety 



874. 



873. 



Fig. 87.3, Platyceras dumosmn ; Meek. Fig-. 8T4, Goniatites mithrax ; Hall. 



of G. discoideus H. being reported from Ohio. The Goniatites differ from 

 species of the Nautilus family in having the siphuncle ventral, and the mar- 

 gin of the septa deeply flexed. Tentaculites of large size also occur, and the 

 related genus StylioUna (Fig. 916). 



6. Crustaceans. — Trilobites of the Lower Silurian genera, Calymene and 

 Dalmanites, and of the Upper Silurian, Homalonotus, Lichas, Phacops, 

 Proetus, Cyphasjyis, are most common. Under some genera there is a large 

 diversification of form in ornamented heads and pygidia. The following 

 figures from Hall and Clarke illustrate some of the forms from the beds of 

 the Early and Middle Devonian. Figs. 875, 876 are heads of species of 

 Dalmanites; 877, 878, pygidia of species of the same genus ; 879, the head of 

 an Acidaspis; 880, part of a pygidium of another Acidaspis; 881, 882, heads 

 of species of Lichas; 883, part of a pygidium of a Lichas; 884, pygidium of 

 a Proetus. 



The most extravagant of all is Lichas grandis H., which had a pygidium 

 four inches broad armed with seven thorny spines 11 to 2^ inches long, and a 

 grossly protuberant warty head, with a stout spine by the side of each 

 eye. The extremity of the pygidium, restored from an imperfect specimen, 

 is represented in Fig. 883. In contrast with these, other species of Dalman- 

 ites and Lichas are very plain. Those of Proetus (Fig. 884) are all prim- 

 looking species, with evenly curving outline before and behind. 



Crustaceans of the Phyllopod and Ostracoid types are rare. But Barnacles 

 of a peculiar kind occur, imbedded to their upper surfaces in the Corals of 



