234 DAVIS : DISTRIBUTION OP FOSSIL FISHES. 
water types. Being associated in death, they must have lived 
together, and the question naturally follows as to whether the 
marine forms adapted themselves to the environment of the fresh- 
water species or vice versa. There is the further consideration, 
that though the recent fishes live for the most part in their 
respective media and are incapable of adapting themselves to a 
different one ; this may not always have been the case, and it is 
equally possible that during the deposition of the coal measures 
the sharks may have been fresh- water fishes, or the ganoids may 
have been marine. Amongst living fishes, the sharks are a large 
and numerous group, for the most part existing in the sea, but 
occasionally becoming more or less adapted to living in fresh- 
water. It is well-known that sharks of large size ascend many 
rivers such as the Ganges. The saw-fish has been observed in 
many rivers of both Africa, Asia, and the South Sea Islands ; and 
sharks have been seen as far inland as Bagdad, in the river Euph- 
rates, by Capt. Sleigh and others. There is also the instance narrated 
by the late Thos. Belt, of sharks inhabiting the Lake Nicaragua in 
Central America, completely cut off from all communication with 
the sea. These cases prove that even at the present time the 
Elasmobranchs have the power to adapt themselves to living 
either in salt or fresh- water. If we turn to the Ganoids we find that 
with the exception of the Sturgeons, they are all fresh- water fish, 
and do not possess any power or exhibit any inclination to migrate 
to the sea. The sturgeons live indiscriminately in either salt, 
brachish, or fresh-water, but of all the ganoid fish they are most 
unlike the fossil forms. Amongst the Siluroids and other Teleos- 
tean forms it is not uncommon to find marine forms ascending 
rivers to spawn or in search of food, but in no instance are the 
fresh-water species known to leave their native habitat and 
descend to the sea. 
During Carboniferous times the most common and universal 
fish appears to have been Megalichthys, a ganoid of large size, 
attaining four or five, or even eight to ten feet in rare instances, 
in length, protected by thick rhomboidal scales and plates of 
