160 LIVING FOSSILS. 



been ascertained recently that there are certain slight 

 differences which justify the separation of the living species 

 as a distinct genus. At present we have no decisive 

 evidence of the existence of these fishes between the upper 

 Jurassic of the United States and the Pleistocene of 

 Queensland, so that there is a long gap in their history to 

 be filled up, it may be hoped, by future discoveries. 



The Australian lung-fish is, however, but one of three 

 nearly allied genera, of which the other two (Lrpidosiren 

 and Protopterus) are each represented by a single species, 

 severally inhabiting the Amazon and the rivers of West 

 Africa. These three widely separated forms are, then, the 

 sole living representatives of an extensive order which was 

 once widely distributed over the globe, and has been slowly 

 waning ever since the Palaeozoic period. The group is one 

 of especial interest, since from some of its extinct re- 

 presentatives, or nearly allied fishes, it is probable that 

 amphibians, and hence the higher vertebrates, have all 

 been derived. 



Of nearly equal interest with the lung-fishes are the 

 bony pikes (Lepidosteus) of the rivers of North America 

 and the bichir (Polypterus)* of the upper Nile and the 

 rivers of Western Africa ; which, together with another 

 West African form (Calamoichthys), are the sole existing 

 representatives of the mail-clad ganoid fishes so abundant 

 during the Secondary period. The African forms are at 

 present unknown in the fossil state, but the bony pikes 

 date from the lower Eocene, and thus indicate continuity 

 with the extinct Secondary types. 



Another living fossil among fishes is the well-known 

 Port Jackson shark ( Cestracion) , the last survivor of a 

 genus ranging in the Secondary rocks of Europe down to 

 the Kiineridge clay ; and also the sole living member of a 

 vast group of sharks characterized by the pavement of 

 crushing teeth with which the mouth is covered. As 

 remarked long since by Buckland, cestraciont sharks lived 

 side by side with trigonias in the old Jurassic seas of 

 Europe ; and it is not a little remarkable to find the 



* See page 145. 



