ELASMOBRANCHII. 345 



From a study of the skull, it becomes apparent at once why 

 in fossil teeth of Ceratodus nothing or very little of the bone 

 attached to them has been preserved. These teeth rest on 

 cartilage as well as on bone, the latter being a very thin and 

 porous layer which could not be preserved, unless the pro- 

 gress of stratification had been going on with as little disturb- 

 ance as in the Solenhofen Schiefer ; but the matrix in which 

 fossil Ceratodont teeth are found shows that it was formed 

 under very different conditions, and it is certainly not of a 

 nature to permit the supposition that thin porous lamellae of 

 bone would have been preserved entire. 



" The structure of the skeleton reminds us much of that of the 

 sturgeons, Chimsera, and especially of Lepidosiren ; and of all 

 the modifications by which it differs from these types, perhaps 

 none is of greater interest than that observed in the paddles. 

 The central part of the paddle, which we have found externally 

 to be covered with scales, is supported by a jointed axis of 

 cartilage extending from the root to the extremity of the pad- 

 dle ; each joint bears a pair of three- or two- or one-jointed 

 branches. This is the case in the hind as well as fore paddles, 

 and we are justified in supposing that those extinct Ganoids of 

 which impressions of paddles with scaly centres have been 

 preserved, were provided with a similar internal skeleton." 



Upon the whole, Dr. Giinther concludes : i. That the Bar- 

 ramunda is not generically separable from the almost exclusively 

 Triassic genus Ceratodus, which was founded simply upon de- 

 tached teeth ; 2. That the Barramunda is very closely allied to 

 certain of the Crossopterygious Ganoids, such as the Dipterus 

 of the Old Red Sandstone, the chief difference being, that the 

 tail of the latter is heterocercal ; 3. That the order Dipnoi 

 should be considered merely as forming a sub-order of the 

 Ganoidei; 4. That the Ganoidei may be united with the Elas- 

 mobranchii into a single group, which may be termed Palceich- 

 thyes, and which is characterised by having a " heart with a 

 contractile bulbus arteriosus, intestine with a spiral valve, and 

 optic nerves non-decussating ; " 5. That the Ganoidei are the 

 Fresh-water Palceichthyes, and the Elasmobranchii are the 

 Marine Palczichthyes. 



If the views of this high authority be ultimately adopted, it 

 will have to be admitted that the Dipnoi, /Instead of being un- 

 known in a fossil state, have enjoyed a- vast antiquity, dating 

 their existence from the Lower Old Red Sandstone. 



