302 LOWER VERTEBRATES. 



These three genera, Ceratodus, Protopteriis, and Lepidosireti, have a very wide 

 distribution, a fact which here, as elsewhere, indicates that the group is a very ancient 

 one, and so we may expect to find remains of similar forms in the older rocks. We 

 have already referred to the fossil teeth of Ceratodus from the mesozoic strata, but 

 apparently the lung-fishes extend back to an earlier period, and some authorities 

 claim that the Devonian fishes, Dipterus and Phaneropleuron, belong to the class of 

 Dipnoi. These ancient forms were much more fish-like than their modern representa- 

 tives, and seemed to occupy a position, in some respects, intermediate between the 

 ganoids and the lung-fishes of to-day. 



J. S. KiNGSLKT. 



