48 DAVIS! FISH-REMAINS IN THE COAL-MEASURES. 
Mr. Belt be thoroughly authentic, it is one of considerable 
importance. The lake, only separated by a strip of land from 
the Pacific, but not at present connected with that ocean, was 
probably in times past at a lower level, and formed an arm of 
the sea or bay, with a marine Fauna, no doubt including 
amongst other fish the sharks. As the land was elevated, 
and the sheet of water became isolated, it would become a 
salt-water lake in the first instance; but being fed by streams 
from the land, would gradually become less and less salt, 
until no trace of its marine origin remained. It will be a fair 
supposition, that the sharks, and perhaps other forms of 
animal life, would become denizens of the lake when it was 
first separated from the sea; and the inference necessarily 
follows, that as the lake became a fresh- water one, the sharks 
adapted themselves to their gradually changing environment, 
and are now existing in water without a trace of its marine 
origin. 
If the supposed advent of the Nicaraguan sharks in the 
way sketched out be a reasonable one, as it appears to me, it 
leads to a probable corollary with respect to the occurrence 
of Elasmobranchs in association with the remains of the fresh- 
water Ganoids in the shales forming the roofs of the coal 
seams. It would not be an extraordinary supposition, that 
during the many and repeated elevations and submersions of 
the land during the Carboniferous Period, lakes originally 
containing salt water may have been changed to fresh water 
ones in the same manner as Lake Nicaragua; and fish, or other 
living organisms, have become adapted to live in either. A 
strong argument in support of the Elasmobranchs being at 
that time denizens of fresh water, may be adduced from the 
frequent occurrence of the remains of Labyrinthodonts and 
Dipnous fishes, in association with those of the sharks and 
Ganoids. The Labyrinthodonts were air breathers, and 
adapted for living on the muddy shores of rivers or lakes, as 
