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DAVIS : DISTRIBUTION OF FOSSIL FISHES, 
met with in others. Cteuodus is a genus closely allied to Dipterus 
of the old red sandstone, and serves as the connecting link during 
the carboniferous period, between that genus and Ceratodus, which 
still continues to exist. These, along with the recent Lepidosiren 
and Protopterus form a peculiar group of fish with acutely lobate 
paired fins. Their internal organization allies them closely with 
the ganoids, but they differ from the ganoids in possessing a 
modified air-bladder which enables them to breathe air, and so to 
exist out of water for long periods. These characters have been 
considered of sufficient importance by some naturalists to necessitate 
the creation of the order Dipnoi for their accommodation, whilst 
others taking their otherwise close resemblance to the ganoids into 
account, place them as a branch of that order. It is very probable 
that the latter may turn out their true position. The possession of 
an air-bladder, osseous or otherwise, is not restricted to these five 
genera, but is also to be found in many of the Siluroid Teleosteans, 
as well as the Lepiclosteous and other ganiods, and in the Sirenoid 
fishes of the rivers of South America. 
Amongst the fishes comprised in the Elasmobranchii the 
genera Ctenacanthus and Gyracanthus present features indicating 
that they attained to a very large size. The only known fossil 
parts of these fishes are the spines, analogous to those of the 
spiny dog-fish of our coasts, in which the spines are placed before 
each of the dorsal fins. Teeth and dermal tubercles have been at 
various times associated with the spines, but without conclusive 
proof that they belonged to the same genus of fishes. The spines 
of both genera are tolerably common and occur in greater or less 
abundance in every stratum where fish remains have been found. 
They are sometimes as much as two and a half to three inches in 
diameter and a couple of feet long. Along their posterior surface 
there is a deep groove to which the fin was attached. Some of 
the spines are worn at their distal ends and appear to have been 
attached to the pectoral fins, being worn by rubbing against the 
bottom, they indicate a fish which was a ground feeder and 
accustomed to a position on or near the bottom. 
