504 MANUAL OF ZOOLOGY. 



CHAPTER LVI. 

 DISTRIBUTION OF FISHES IN TIME. 



THE geological history of fishes presents some points of pe- 

 culiar interest. Of all the classes of the great sub-kingdom 

 Vertebrata, the fishes are the lowest in point of organisation. 

 It might therefore have been reasonably expected that they 

 would present us with the first indications of vertebrate life 

 upon the globe ; and such is indeed the case. After passing 

 through the enormous group of deposits known as the Lauren- 

 tian, Huronian, Cambrian, and Lower Silurian formations 

 representing an immense lapse of time, during which, so far 

 as we yet know with certainty, and leaving the " Conodonts " 

 out of sight, no vertebrate animal had been created we find 

 in the Upper Silurian rocks the first traces of fish. The ear- 

 liest of these, in Britain, are found in the base of the Ludlow 

 rocks (Lower Ludlow Shale), and belong to the Placoganoid 

 genus Pteraspis. Also in the Ludlow rocks, but at the summit 

 of their upper division, are found fin-spines and shagreen, 

 probably belonging to Cestraciont fishes that is to say, to 

 fishes of as high a grade of organisation as the Elasmobranchii. 

 So abundant are the remains of fishes in the next great geo- 

 logical epoch namely, the Devonian or Old Red Sandstone 

 that this period has frequently been designated the " Age of 

 Fishes." Most of the fishes of the Old Red Sandstone belong 

 to the order Ganoidei, but the order Dipnoi appears to be also 

 represented. In the Carboniferous and Permian rocks which 

 close the Palaeozoic period, most of the fishes are still Ganoid, 

 but the former contain the remains of many Elasmobranchii. 

 At the close of the Palaeozoic and the commencement of the 

 Mesozoic epoch, the Ganoid fishes begin to lose that predom- 

 inant position which they before occupied, though they con- 

 tinue to be represented through the whole of the Mesozoic 

 and Kainozoic periods up to the present day. The Ganoids, 

 therefore, are an instance of a family which has endured 

 through the greater part of geological time, but which early 

 attained its maximum, and has been slowly dying out ever 

 since. Towards the close of the Mesozoic period (in the Cre- 

 taceous period) the great family of the Teleostean or Bony 

 Fishes is for the first time known certainly to have made its 

 appearance. The families of the Marsipobranchii and Pharyn- 

 gobranchii have not left, so far as is known, any traces of their 



