Marvels of the Universe 743 
Reference has already been made to the 
existence in the Old Red Sandstone of fishes 
linking in a measure the lung-fishes with 
the more highly-organized fishes known 
as the Teleostomi, of the present day. 
These are the Crsussopterygians, or Thick- 
finned Fishes, which have many points in 
common with two existing genera of African 
river-fish, namely, the Reed Fish, from 
Senegambia and Calabar, and the Many- 
finned Fish of the Nile. These two fishes 
stand quite by themselves amongst living 
forms; and, like the mud-fishes above 
referred to, are evidently the sole survivors 
of a group which in the past was repre- 
sented by a host of interesting and diver- 
sified types; some of the best known 
having been discovered in the Old Red 
Sandstone of Scotland. 
Of the shark-like fishes of the Old Red 
Sandstone, one of special interest claims 
attention This is Climatius, and _ the 
importance of this fish arises from the 
circumstance that it throws some _ light 
upon the origin of the paired fins of other 
fishes, and, incidentally, of the limbs of the 
higher vertebrates. It has already been 
explained that the limbs of the latter 
correspond to the two pairs of fins, called 
the pectoral and the pelvic, which are 
present in all typical fishes. In other words, 
since the higher vertebrates were derived 
from fishes, fin-like limbs were the primitive 
type of the human arms and legs. But 
whence came the fin-like limbs? Many 
facts in the structure and development of 
fishes suggest that the earliest types 
developed a continuous flap-like fin along 
each side of the body, which served to 
balance it in the water and was in all 
respects similar to the unpaired fins running 
along the back and on the tail, both above 
and below. At two points, one in front 
and one behind, the lateral fins became 
differentiated and enlarged, giving rise at 
a very early stage in the evolution of fishes 
FISHES OF THE OLD RED SANDSTONE. 
This fish, the Pterichthys, or Winged Fish, was enveloped in a 
great buckler of overlapping plates and a composite sub-circular head- 
shield. The most curious features, however, are the jointed limb-like 
appendages, which were certainly used for locomotion, but exactly 
how it is impossible to determine. 
to the paired fins, as the intervening portion of the fin in question disappeared as a separate 
structure. Now the interest of Climatius 
is this. It shows unmistakable signs of having 
possessed a series of at all events five paired fins, or rather of spines representing them, for 
