212
THE BIOLOGY OF FISHES
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.Cephalaspis, ‘Arthrodirans, Birkenia, etc.—as belonging,
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so to. speak, to a different and earlier world. Bashford
Dean places them even before
Protochordata, and
for him the class Pisces begins with Acanthodes and
Cladoselache.
This is the logical c0urse, but it cannot
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disguise the fact that the Arthrodirans, Palæospondylus, and »
many others were ossied and were shes.
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It seems better to accept the conclusion, that the Elasmo!
branchs came later.
It may also be taken as certain, that
the armoured
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Ganoids ”
or sedentary Teleosts were not
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the primary forms, and the choice lies between the true
Tèleosts and the Dipnoi. Since the Arthrodirans show a
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mixture of the characteristics of both groups, we are clearly
right lines; but the term Dipnoi refers to a special
condition of {the air—sacs in the modern forms, which was
probably not so well developed in the earliest, and the term
Teleost is c0mprehenSive enough to embrace both as well
as the Gaoids, as suggested by'Bridge (1904). To avoid
confusion, however, it is best to group them together under
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the wider term Osteichthyes.
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The evidence that the eatliest shes, even Palæo,
spondylùs, possessed ossicatios of some sort, cannot, be
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gainsaid; This ossication may have been, probably Was”,
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extremely thin and brOus, like that of the larval -Clupeids
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of the present day, butit came as a natural consequence of a
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body moving pdagicallÿ_ through; the 'Wa'er.
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these early
hadhdfpai‘i‘çd'S and nomed1an
ns eXcept the caudal
:
,delte gill
_ arches have
features
thà_t
_understood5fromïa consideration of the
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Clupéidlärva…g
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j _g Taking
as representing the:
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{
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most prrrmtrveform
some of the changes .
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havetakenplace in the principal characters may be
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—.
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hasvariedin tWo directions: ‘
more _elongated With-the deVeloprhent—_ of
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or long Single —dors‘a1 na-ridæpairîefd ns in the
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