4.02 MANUAL OF ZOOLOGY. 



Upon the whole, Dr Giinther concludes that the Dipnoi are 

 to be regarded as a simple sub-order of Ganoids, and that the 

 entire order Ganoidei may be united with the Elasmobranchii 

 into a single order, called Palmichthyes, characterised by having 

 a " heart with a contractile bulbus arteriosus, intestine with a 

 spiral valve, and optic nerves non-decussating.'' 



CHAPTER LIX. 



DISTRIBUTION OF FISHES IN TIME. 



The geological history of fishes presents some points of peculiar 

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

 brata, 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 Laurentian, Huro- 

 nian, Cambrian, and Lower Silurian formations — ^representing 

 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 Palseozoic 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, 

 are an instance of a family which has endured through the 

 greater part of geological time, but which early attained its 



