Mesozoic rocks than
the Brachyura are.

None of the cases of progressive modification which are cited from
among the Invertebrata appear to me to have a foundation less open to
criticism than these; and if this be so, no careful reasoner would,
I think, be inclined to lay very great stress upon them. Among the
Vertebrata, however, there are a few examples which appear to be far
less open to objection.

It is, in fact, true of several groups of Vertebrata which have lived
through a considerable range of time, that the endoskeleton (more
particularly the spinal column) of the older genera presents a less
ossified, and, so far, less differentiated, condition than that of the
younger genera. Thus the Devonian Ganoids, though almost all members of
the same sub-order as 'Polypterus', and presenting numerous important
resemblances to the existing genus, which possesses biconcave vertebrae,
are, for the most part, wholly devoid of ossified vertebral centra. The
Mesozoic Lepidosteidae, again, have, at most, biconcave vertebrae, while
the existing 'Lepidosteus' has Salamandroid, opisthocoelous, vertebrae.
So, none of the Paleozoic Sharks have shown themselves to be possessed
of ossified vertebrae, while the majority of modern Sharks possess
such vertebrae. Again, the more ancient Crocodilia and Lacertilia
have vertebrae with the articular facets of their centra flattened
or biconcave, while the modern members of the same group have them
procoelous. But the most remarkable examples of progressive modification
of the vertebral column, in correspondence with geological age, are
those afforded by the Pycnodonts among fish, and the Labyrinthodonts
among Amphibia.

The late able ichthyologist Heckel pointed out the fact, that, while
the Pycnodonts never possess true vertebral centra, they differ in the
degree of expansion and extension of the ends of the bony arches of
the vertebrae upon the sheath of the notochord; the Carboniferous forms
exhibiting hardly any such expansion, while the Mesozoic genera present
a greater and greater development, until, in the Tertiary forms, the
expanded ends become suturally united so as to form a sort of false
vertebra. Hermann von Meyer, again, to whose luminous researches we
are indebted for our present large knowledge of the organization of the
older Labyrinthodonts, has proved that the Carboniferous 'Archegosaurus'
had very imperfectly developed vertebral centra, while the Triassic
'Mastodonsa