378 MANUAL OF ZOOLOGY. 



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 Cretaceous 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, Pharyngobranchii, and Dipnoi, have 

 not left, so far as is known, any traces of their existence in 

 past time. Judging from analogy, however, it is highly pro- 

 bable that the two former of these must have had a vast anti- 

 quity, and it is not impossible that the so-called " Conodonts " 

 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. 



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. 140), Cephalaspis (fig. 139), 

 Pteraspis, and Coccosteus (fig. 140). The Lepidoganoid division 

 of the order is now also abundantly represented for the first 

 time, the genera Dipterus, Osteolepis (fig. 138), Glyptolepis y 

 Holoptychius, Diplacanthus, and many others belonging to this 

 section. As regards the further distribution of the Placogan- 

 oids, 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 Sturionida, 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 

 they are still represented, but all the forms are as yet heterocer- 

 cal. In the Oolitic Rocks, for the first time, Lepidoganoids with 



