280 PALiEOZOIC TIME — DEVONIAN AGE. 



York westward became to a large extent covered with coral-grow- 

 ing seas. The wide distribution of the rocks proves the vast area 

 of those coral seas. It also teaches that they were shallow seas ; 

 for, as already remarked, corals grow and form limestones only 

 where they are within the reach of the waves. The Upper Hel- 

 derberg period was eminently the coral-reef period in Palaeozoic 

 history. 



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

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

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

 corals and Brachiopods (Am. Jour. Sci. [2] xxvi. 120). Among the 

 former, besides the Favosites Gothlandica (Upper Silurian in Europe), 

 there are Heliolites porosa, and Cyathophyllum helianthoides, Devonian 

 species occurring in Europe and America. 



This identity of species between the Arctic, and Europe and 

 America, just illustrated, favors an approximate identity of climate : 

 there is no sufficient evidence of a cold Arctic, or even of a wide 

 diversity of zones. 



HAMILTON PERIOD (10). 



Epochs. — 1. Marcellus, or that of the Marcellus shale (10 a) ; 

 2. Hamilton, or that of the Hamilton beds (10 b) ; 3. Genesee, or 

 that of the Genesee shale (10 c). 



474, 475). Fig. 475 a shows the under surface of 475, and illustrates the manner 

 in which the scales are locked together. Among them there are (1) the Cepha- 

 laspids, having a broad buckler-head ; Ex., Cephalaspis (fig. 517); and (2) the 

 Sauroids (figs. 473 and 519), having the form of ordinary fishes, and sharp, 

 though sometimes very small, teeth. Figs. 479, 480 are teeth (nat. size) of a 

 Lepidosteus, and a Cricodus. Other genera of this group are Dipterus, Gheiro- 

 lepis, Palseoniscus, etc. The Lepidoids of Agassiz are here included. 



(b.) Imbricates. — Scales not rhombic, imbricate or set on like shingles, as in 

 Holoptychius (fig. 518) and the modern Amia. 



The Pycnodonts are Rhombifers, having smooth pavement-teeth. Fig. 478 

 represents part of the pavement-teeth in a species of Gyrodus. 



C. Acipenseroids. — Include the modern Sturgeon, which has a cartilagi- 

 nous skeleton, large rounded plates, and no teeth. 



Labyrinthine structure of Ganoid teeth. — The teeth in the Sauroids, or at least 

 in many of them, have a labyrinthine structure within. This structure is illus- 

 trated in one of its simplest forms in fig. 481, which is an enlarged section, by 

 Agassiz, of the tooth of a living Lepidosteus. It consists in a folding inward of 

 the enamel and dentine. In fig. 480 the strias of the tooth correspond to these 

 inward foldings. 



