184 THE SUCCESSION OF ANIMAL FORMS 



They are unfortunately so comminuted as to resemble the 

 debris of the food of some larger animal ; but in so far as I can 

 judge from specimens kindly given to me, 1 they resemble the 

 bony coverings of some of the familiar fishes of the Devonian. 

 Thus they would indicate, with Pander's and Rohan's speci- 

 mens, already two distinct types of fishes as existing almost as 

 early as the higher invertebrates of the sea. 



In the Silurian (Upper Silurian of Murchison) we have un- 

 doubted evidence of the same kind, on both sides of the 

 Atlantic, in teeth and spines of sharks, and the plates which 

 protected the heads and bodies of the plate-covered fishes 

 (Placo-ganoids). But it is in the Devonian that these types 

 appear to culminate, and we have added to them that remark- 

 able type of " lung fish," as the Germans call them, represented 

 in our modern world only by the curious and exceptional 

 Burramunda of Australia, and the mud fishes of Africa and 

 South America,* creatures which show, as do some of the 

 mailed fishes, or ganoids, of equally great age, the intermediate 

 stages between a swimming bladder and a lung, and thus ap- 

 proach nearer to the air-breathing animals than any other fishes. 



Many years ago, in "Acadian Geology," I referred to the 

 probability that the mailed and lung fishes of the Devonian and 

 Carboniferous possessed airb ladders so constructed as to 

 enable them to breathe air, as is the case with their modern 

 representatives. In the modern species this, no doubt, enables 

 them to haunt badly aerated waters, in swamps and sluggish 

 streams, and in some cases even to survive when the water 

 in which they live is dried up. In the Carboniferous and 

 Devonian it may have served a similar purpose, fitting them 

 to inhabit the lagoons and creeks of the coal swamps, the 

 water of which must often have been badly aerated. It makes 

 against this that some sharks followed them into these waters, 



1 By Mr. F. D. Adams and Dr. Walcott. 

 2 Ceratodus, Lipidosiren, Protopterus. 



