50 



strata, species occui'ing in America being identical wth those of 

 the British rocks of contemporaneous age. The dentition of the 

 Devonian Di^Jtems is also closely related to that of Ceratodus, as 

 well as Lepidosiren. 



Thus the history of the Dqmol, an order before the discovery 

 of the Australian Ceratodiis only represented by the mud fishes of 

 Africa and South America, is carried back to remote geological 

 ages, and the four li\-ing representatives, at present kno^mi, are 

 found to be the survivors of a well defined and characteristic 

 group of fishes first appearing in the Devonian age. They can be 

 traced up from Dipteriis, through the carboniferus Ctenodus, to the 

 Jurassic Ceratodonts, and then the link is lost sight of until their 

 lineal descendants reappear widely distributed on the surface of 

 the present world. This is but an illustration of the truth that 

 species which have the greatest vertical range in time have also 

 the Avidest geographical distribution, or that a wide distribution 

 proves the antiquity of the genus. It is certainly a very 

 significant fact that the group of living fish most closely allied to 

 the ampliibian reptiles should be represented in the Devonian 

 rocks, long before the most sunply constructed amphibians 

 appeared on the scene of life in the swamps of the carboniferous 

 period. 



The Dipnoi, as at present constituted, comprises the following 

 families : Frotopteriiia, Ceratodmitina, Ctemdixlipteridoi, and possibly 

 Fhaiieroplcuridce. They are closely allied to the Ganoids, and 

 especially to that sub-order termed, by Professor Huxley, the 

 Crossopterygidce, or " fringe-finned, " to be presently referred to. 

 Dr. Giinther, indeed, proposes to unite the dipnoids with the 

 ganoids, as a distinct family ; but Professor. Huxley considers 

 that, tJiough nearly related to that order, they yet possess many 

 important diff"erences. 



It seems as if the Dijmoi had also some aflinities with the 

 group of fishes known as Placodcrms, for a most remarkable fossil 



