Similarly, the Univalves with
breathing-tubes and a corresponding notch in the mouth of the
shell ("siphonostomatous" Univalves) are regarded as higher in
the scale than the round-mouthed vegetable-eating Sea-snails, in
which no respiratory siphons exist ("holostomatous" Univalves);
but the latter abound in the Palaeozoic rocks--whereas the former
do not make their appearance till the Jurassic period, and their
higher groups do not seem to have existed till the close of the
Cretaceous. The _Cephalopods_, again--the highest of all the groups
of Mollusca--are represented in the Palaeozoic rocks exclusively
by Tetrabranchiate forms, which constitute the lowest of the
two orders of this class; whereas the more highly specialised
Dibranchiates do not make their appearance till the commencement
of the Mesozoic. The Palaeozoic Tetrabranchiates, also, are of
a much simpler type than the highly complex _Ammonitidoe_ of
the Mesozoic.
Similar facts are observable amongst the _Vertebrate animals_.
The Fishes are the lowest class of Vertebrates, and they are
the first to appear, their first certain occurrence being in
the Upper Silurian; whilst, even if the Lower Silurian and Upper
Cambrian "Conodonts" were shown to be the teeth of Fishes, there
would still remain the enormously long periods of the Laurentian
and Lower Cambrian, during which there were Invertebrates, but
no Vertebrates.
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