DEVONIAN AGE. 265 



The Osseous Jishes or Teliosts, which include nearly all modern kinds, 

 except the Sharks and Rays, and have usually membranous scales 

 (like Figs. 516, 517, the former a " Cycloid " scale and the latter a 

 "Ctenoid"), are not known among fossils before the Middle Mesozoic. 



The lowest division of modern fishes includes a few very small kinds, 

 like the Amphioxus, which are scale-less, fin-less, brain-less, without 

 special organs of sense beyond feelers around the mouth, and with 

 the skeleton membranous, and the heart rudimentary. These lowest 

 of Vertebrates, inferior even to the higher Radiates, would naturally 

 be looked for as precursors of the Selachians and Ganoids ; but no 

 remains of them have been found. Had such species existed, however,, 

 they could scarcely have left remains, as they have no hard parts. 



III. General Observations. 



Geography. — In the first epoch of this period, that of the Cauda- 

 galli grit, the beds were, as a body, more easterly in position over 

 New York than those of the preceding period. In the Schoharie 

 epoch, they were still farther to the east than the Cauda-galli grit ; 

 at the same time, they continue to be sandstones. But with the next 

 epoch there was a change. The continent, from eastern New York 

 westward, became to a large extent covered with coral-growing seas. 

 The wide distribution of the rocks proves the vast area of those coral 

 seas. It also teaches that they were shallow seas ; for so large corals 

 would form limestones only where they were within the reach of the 

 waves. 



The presence of the hornstone, through many layers of the lime- 

 stone, indicates that, over the bottom, where mollusks and other species 

 were living and making the material for the limestones, there were 

 often also Sponges and Diatoms or Polycystines, making microscopic 

 siliceous shells and spicules ; so that, while the calcareous sands of the 

 former were solidifying into limestone, the microscopic grains of silica 

 became aggregated here and there into siliceous concretions or masses 

 of hornstone. 



Climate — The question of the occurrence of rocks of this period 

 in the Arctic region is not yet decided. It is probable that they exist 

 there, on North Somerset and elsewhere, judging from the fossil corals 

 and Brachiopods. 1 Among the former, besides the Favosites Goth- 

 landica (Upper Silurian in Europe), there are Heliolites porosa and 

 Cyathophyllum helianthoides, Devonian species common to both Europe 

 and America. 



This identity of species between Arctic lands and Europe and 

 1 Am. Jmir. Sci., II. xxvi. 120. 



