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 biconclave 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 Palaeozoic 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