sutures exhibiting a certain increase of complexity in the newer
genera. Here, however, one is met at once with the occurrence of
'Orthoceras' and 'Baculites' at the two ends of the series, and of the
fact that one of the simplest Genera, 'Nautilus', is that which now
exists.

The Crinoidea, in the abundance of stalked forms in the ancient
formations as compared with their present rarity, seem to present us
with a fair case of modification from a more embryonic towards a less
embryonic condition. But then, on careful consideration of the facts,
the objection arises that the stalk, calyx, and arms of the paleozoic
Crinoid are exceedingly different from the corresponding organs of a
larval 'Comatula'; and it might with perfect justice be argued that
'Actinocrinus' and 'Eucalyptocrinus', for example, depart to the full as
widely, in one direction, from the stalked embryo of 'Comatula', as
'Comatula' itself does in the other.

The Echinidea, again, are frequently quoted as exhibiting a gradual
passage from a more generalized to a more specialized type, seeing that
the elongated, or oval, Spatangoids appear after the spheroidal
Echinoids. But here it might be argued, on the other hand, that the
spheroidal Echinoids, in reality, depart further from the general plan
and from the embryonic form than the elongated Spatangoids do; and that
the peculiar dental apparatus and the pedicellariae of the former are
marks of at least 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