st as great differentiation as the petaloid ambulacra and
semitae of the latter.

Once more, the prevalence of Macrurous before Brachyurous Podophthalmia
is, apparently, a fair piece of evidence in favour of progressive
modification in the same order of Crustacea; and yet the case will not
stand much sifting, seeing that the Macrurous Podophthalmia depart as
far in one direction from the common type of Podophthalmia, or from any
embryonic condition of the Brachyura, as the Brachyura do in the
other; and that the middle terms between Macrura and Brachyura--the
Anomura--are little better represented in the older 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 an