DISTRIBUTION OF FISHES IN TIME. 377 



zon, and the Z. annectens from the Gambia. They both inhabit 

 the waters of marshy tracts, and appear to be able in the dry 

 season to bury themselves in the mud, forming a kind of 

 chamber in which they remain dormant till the return of the 

 rains. Recently there has been discovered in the rivers of 

 Queensland (Australia) a fish which has been described under 

 the name of Ceratodus (?) Fosteri, and which would appear to 

 be very closely related to the Lepidosiren. 



CHAPTER LIX. 

 DISTRIBUTION OF FISHES IN TIME. 



THE geological history of fishes presents some points of pecu- 

 liar 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 repre- 

 senting an immense lapse of time during which, so far as we yet 

 know, no vertebrate animal had been created, we find in the Upper 

 Silurian Rocks the first traces of fish. The earliest of these, in 

 Britain, is found in the base of the Ludlow Rocks (Lower Lud- 

 low Shale), and belongs 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 geological 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 Gan- 

 oidei. 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 Plagiostomous fishes. 

 At the close of the Palaeozoic and the commencement of the 

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

 nant position which they before occupied, though they continue 

 to be represented through the whole of the Mesozoic and Kain- 

 ozoic periods up to the present day. The Ganoids, therefore, 



