CHAPTER XVI 
THE CROSSOPTERYGII 
mwILASS Teleostomi.—We may unite the remaining groups 
BN) of fishes into a single class, for which the name Teleos- 
tom (reAeos, true; crou“a, mouth), proposed by Bona- 
parte in 1838, may be retained. The fishes of this class are 
characterized by the presence of a suspensorium to the man- 
dible, by the existence of membrane-bones (opercles, sub- 
orbitals, etc.) on the head, by a single gill-opening leading to 
gill-arches bearing filamentous gills, and by the absence of 
claspers on the ventral fins. The skeleton is at least partly 
ossified in all the Teleostom:. More important as a primary 
character, distinguishing these fishes from the sharks, is the 
presence typically and primitively of the air-bladder. This 
is at first a lung, arising as a diverticulum from the ventral side 
of the cesophagus, but in later forms it becomes dorsal and is, 
by degrees, degraded into a swim-bladder, and in very many 
forms it is altogether lost with age. 
This group comprises the vast majority of recent fishes, 
as well as a large percentage of those known only as fossils. 
In these the condition of the lung can be only guessed. 
The Teleostomt are doubtless derived from sharks, their 
relationship being possibly nearest to the Ichthyotomi or to the 
primitive Chimeras. The Dipnoans among Teleostomi retain 
the shark-like condition of the upper jaw, made of palatal 
elements, which may be, as in the Chimera, fused with the cra- 
nium. In the lower forms also the primitive diphycercal or 
protocercal form of tail is retained, as also the archipterygium 
or jointed axis of the paired fins, fringed with rays on one or 
both sides. 
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