428 MANUAL OF ZOOLOGY. 



from the Lower Silurian rocks of Russia may yet be shown to 

 be the horny teeth of fishes allied to the Lampreys. At pre- 

 sent, however, the weight of evidence is in favour of looking 

 upon these problematical little bodies as probably referable to 

 some of the Invertebrata. 



It is also to be borne in mind that, though it has not yet 

 been possible to definitely refer any fossil fishes to the order 

 of the Dipnoi, recent discoveries have rendered it extremely 

 probable that some well-known extinct types really belong to 

 this order. Thus the great " Barramunda" (Ceratodus Fosteri] 

 of the rivers of Queensland would seem to be truly referable 

 to the Triassic genus Ceratodus, in which case this latter must 

 be removed to the Dipnoi. This remarkable fish also presents 

 some striking points of resemblance with certain extinct 

 Ganoids, such as Dipterus. Upon the whole, therefore, there 

 are good grounds for accepting Dr Giinther's suggestion that 

 the Dipnoi should be regarded as a mere sub-order of the 

 Ganoids. 



Leaving these unrepresented orders out of consideration, 

 the following are the chief facts as to the geological distribu- 

 tion of the other great groups : 



I. Ganoidei. As far as is yet known with certainty, the 

 oldest representatives of the fishes belong to this order. The 

 order is represented, namely, in the Upper Silurian rocks 

 by the remains of at least four genera. In the Devonian 

 rocks, or Old Red Sandstone, the Ganoids attain their 

 maximum both in point of numbers and development. The 

 Placoganoid division of the order is represented by the 

 singular genera Pterichthys (fig. 177), Cephalaspis (fig. 176), 

 Pteraspis and Coccosteus (fig. 177). The Lepidoganoid division 

 of the order is now also abundantly represented for the first 

 time, the genera Dipterus, Osteolepis (fig. 175), Glyptolcpis, 

 Holoptychius, Diplacanthus, and many others belonging to this 

 section. As regards the further distribution of the Placoga- 

 noids, the section of the Ostracostei, characterised by the great 

 development of the cephalic buckler, appears to have died 

 out at the close of the Devonian period. The other section, 

 however namely, that of the Sturionidce is represented in 

 the Liassic period (Mesozoic) by the genus Chondrosteus, and 

 in the Eocene (Kainozoic] by a true Sturgeon, the Acipenser 

 toliapicus. 



The Lepidoganoids continue from the period of the Old Red 

 in great profusion, and they are represented by very many 

 genera in the Carboniferous and Permian rocks. In the earlier 

 portion of the Mesozoic period i.e., in the Lias and Trias 



