262 Dr Hibbert on the Limestone of Burdiehouse, 



the period when the Megalichthys lived ; — to movements by 

 which land rose from some large lake, only to be again submerged, 

 and by which alternations of submersion and emersion were often 

 repeated. During such a state of our planet, lungs might have 

 been given to sauroid fish, with the view of enabling them to 

 support for a continuance the changes of element to which they 

 were liable ; — that when they were left dry, they might be en- 

 abled to maintain life until again floated, — either by rains, or by 

 the renewed effect of geological agency. 



There is also another circumstance to be kept in view, name- 

 ly, that as the remains of the Megalichthys are found in bitu- 

 minous shale, and even in coal itself, it is evident that the animal 

 must have frequented shallows, and wet marshes, among which he 

 would have been liable to be left dry, and consequently to perish, 

 unless for the provision of lungs. 



That fish with analogous habits exist at the present day, may 

 be learned from Dr Hamilton's account of the scaly Cobojius of 

 India, which lives among the extensive marshes of the Yasor 

 district. This fish is possessed of great tenacity of life in the 

 open air ; he is enabled to five many days without water ; and he 

 is even endowed with a considerable facility of progressive mo- 

 tion on land. 



NOTES TO SECTION X. 



A speculation has been recently thrown out, which connects itself rather with 

 the character of the watery fluid, than of the atmosphere in which these sauroid fish 

 lived. Upon this subject, M. Agassiz has remarked, that if structure be indicative 

 of habit, this sauroid tendency of the fish of the limestone of Burdiehouse will lead 

 to further discoveries of the aqueous fluids of the globe, which were neither salt nor 

 fresh ; as, by the character of the vegetation of remote periods, naturalists have 

 been led to deduce a difference in the gaseous constitution of the atmosphere. — 

 (Report of the Edinburgh Meeting of the British Association of Science.) 



With regard to this observation I would suggest, that the argument regarding 

 a half-saline state of waters, considered as prevailing over the whole ancient surface 

 of the globe, is unsatisfactory, so long as it can be shewn that two distinct species of 

 deposits, one apparently fitted for marine, and the other for fresh-water life, have 

 existed. Proofs of this coexistence form the leading object of investigation in the 

 present memoir. 



