610 



HISTORICAL GEOLOGY. 



these plates, two, represented enlarged in Figs. 695, 695 a, are referred to 

 Placoderuis (see page 417), the group which comprises the oldest Fishes 

 previously known, those of the Upper Silurian and early Devonian. The 

 scales, Fig. 696, have the markings of a tyi)ical Ganoid, much like those of 

 the genus Holoptychius, a form not found hitherto in beds earlier than the 

 Middle Devonian. Besides these, there are remains (Figs. 697, 697 a) of 

 what are supposed to be the ossified sheaths of the notochord of a species 

 of the Shark tribe related to the Chimsera (page 416). The beds affording 

 these remains of Fishes contain many other fossils that are referred to 

 the Lower Trenton, and are overlaid by others carrying Upper Trenton 

 fossils. 



2. Utica and Hudson Epochs. 



Graptolites abound in the shales of the Utica and Hudson groups, 

 especially the former. Thirty species or more have been described from 

 the Utica slate, and some of these are represented in Figs. 698-702. 



Graptolites. — Fig. 698, Lasiograptus (Diplograptus) miicronatus ; 699, Coenograptus gracilis; 700, Clima- 

 cograptus bicornis ; 701, 701 a, Diplograptus pristis ; 702, Dicranograptus ramosus. Asteeioid. — Fig. 703, 

 Pateaster Jamesi. Figs. 698-702 from Hall ; 703, J. G. Anthony. 



Corals occur of several genera. Favistella, Fig. 704, is a massive Coral, 

 with crowded stellate cells. Halysites, Fig. 705, grew in vertical plates, in- 

 tersecting one another ; in a transverse section the cells look like the loops 

 of a chain, whence the common name chain coral. Another Coral grew in 



