264 PALAEOZOIC TIME — UPPER SILURIAN. 



all three of the Wenlock; 438, the Pentamerus Knightii of the Aymestry lime- 

 stone ; 439, Grammyaia cingulata of Dudley ; 440, the Pterygotus bilobus, related 

 to the Eurypterus, — a genus some of whose species, according to Salter, were 

 seven or eight feet long, exceeding modern Crustaceans hy several feet. It is 

 an early lesson taught by science, that size, however important in some parts 

 of the system of nature, is no necessary criterion of rank. 



On some shells traces of the original colors still remain, proving 

 that there was beauty of coloring in Silurian times as well as now, 

 and also that the species lived in comparatively shallow waters. 

 Professor Forbes has stated that colors arranged in figures are not 

 found on shells which live below a depth of fifty fathoms. 



Notwithstanding the striking coincidences in species and still more in genera 

 between the New and Old World, there are discrepancies which make it quite 

 difficult to determine with precision the equivalency of the rocks. The Wenlock 

 beds contain some 40 Niagara species; and still there are quite a number 

 (among them Boilmania Hausmanni and Pentamerus galeatus) that are closely 

 related to species found in the period of the Lower Helderberg. The Ludlow 

 beds are related similarly to the Lower Helderberg, and still some species occur 

 in them that in America exist only in the Devonian. This is so striking a fact 

 that the Ludlow is generally regarded as extending into the Devonian, — the 

 American records in this being the assumed standard. Mr. Hall takes this 

 ground. 



The group of Cyathophylloid corals is much more largely de- 

 veloped in the Ludlow beds than in the American Silurian ; for the 

 coral-reef period in America is that of the Upper Helderberg in 

 the Lower Devonian. Besides, there is the higher group of Fishes, 

 the first of the Vertebrates, some relics of which occur in the upper 

 layers of the Ludlow beds. These remains are of the genera Cepha- 

 laspis, Onchus, and Plectrodus : the first belongs to the tribe of Gan- 

 oids, and the other two to that of the Selachians or Sharks. In 

 the same Ludlow beds, traces of land-plants have been found, in 

 the shape of minute globular bodies which have been regarded as 

 the seed-vessels of some species of the Lycopodium tribe of plants. 

 The upper part of the Silurian beds in Bohemia has also afforded 

 fish-remains of similar character to those of the Ludlow beds. 



If the Ludlow beds be divided and the upper part referred to the 

 Devonian, then these species of fishes and the associated land-plants 

 will come into that age, — the Age of Fishes. 



Still, if fossil fishes should hereafter be found even lower in the 

 Silurian, it will harmonize entirely with the system in other parts 

 of the geological series. As has been stated on p. 126, we naturally 

 look for precursors of every Age. There were Mammals before the 

 age of Mammals, Reptiles before the age of Reptiles, Acrogens and 



