586 SUPPOSED PERIOD OF INVERTEBRATE ANIMALS. [Ch. XXVH. 



bites, Corals, and other fossils already obtained from more ancient 

 Silurian formations, Upper, Middle, and Lower, we may well ask 

 whether any set of fossiliferous rocks newer in the series were ever 

 studied with equal diligence and over so vast an area without yielding 

 a single ichthyolite. 



Yet we ought to hesitate before we accept, even on such evidence, 

 so sweeping a conclusion, as that the globe, for ages after it was 

 inhabited by all the great classes of invertebrata, remained wholly un- 

 tenanted by vertebrate animals. In the first place, we must remember 

 that we have detected no insects, or land-shells, or freshwater pulmon- 

 iferous mollusks, or terrestrial crustaceans, or plants (with the excep- 

 tion of fucoids), in rocks below the Upper Silurian. Their absence 

 may admit of explanation, by supposing almost all the deposits of that 

 era hitherto examined to have been formed in seas far from land or 

 beyond the influence of rivers. Here and there, indeed, a shallow- 

 water or even a littoral deposit may have been met with in North 

 Wales or North America ; but, speaking generally, the Silurian de- 

 posits, as at present known, have certainly a more pelagic character 

 than any other of equal extent and thickness. 



It is a curious fact, and not perhaps a mere fortuitous coincidence, 

 that the only stratum in which land-plants occur is also the only one 

 which has yielded the remains of fish in any considerable abundance. 

 Bone-beds in general, such as that of the uppermost Trias at Bristol 

 and Stuttgart, or that of the Carboniferous Limestone near Bristol 

 and Armagh, or, lastly, that of the " Upper Ludlow," are remarkable 

 for containing teeth and bones, much rolled, and implying transporta- 

 tion from a distance. The association of the sporangia of Lycopodi- 

 acese (see p. 552) with the Ludlow fish-bones shows that plants had 

 been washed from some dry land then existing, and had been drifted 

 into a common submarine receptacle with the bones ; and it is well 

 known that in the present state of the globe fish occur in the greatest 

 numbers at the junction of rivers with the sea. Where the Upper 

 Ludlow is devoid of plants, as is usually the case, it is as destitute of 

 ichthyolites as are the Wenlock or Llandeilo beds. 



It has been suggested in explanation, that Cephalopoda were so 

 abundant in the Silurian period that they may have discharged the 

 functions of fish ; to which we may reply that both classes coexisted 

 in the Upper Silurian period, and both of them swarmed together in 

 the Carboniferous and Liassic seas, as they do now in certain parts of 

 the ocean. We may also remark that we are too imperfectly ac- 

 quainted with the distribution of scattered bones and teeth or the 

 skeletons of dead fish on the floor of the existing ocean, to have a right 

 to theorize with confidence on the absence of such relics over wide 

 spaces at any former era. 



They who in our own times have explored the bed of the sea in- 

 form us that it is in general as barren of vertebrate remains as the 

 soil of a forest on which thousands of mammalia and reptiles have 



