goniatites having zigzag, angulated sutures. Late in
the succeeding Carboniferous period appear shells with a truly ammonoid
complexity of sutures, and in the Permian their number and variety
cause them to form a striking element of the marine faunas. It is in the
Mesozoic era, however, that these shells attain their full development;
increasing enormously in the Triassic, they culminate in the Jurassic
in the number of families, genera and species, in the complexity of
the sutures, and in the variety of shell-ornamentation. A slow decline
begins in the Cretaceous, ending in the complete extinction of the whole
group at the end of that period. As a final phase in the history of the
ammonites, there appear many so-called "abnormal" genera, in which the
shell is irregularly coiled, or more or less uncoiled, in some forms
becoming actually straight. It is interesting to observe that some of
these genera are not natural groups, but are "polyphyletic," i.e.
are each derived from several distinct ancestral genera, which have
undergone a similar kind of degeneration.

In the huge assembly of ammonites it is not yet possible to arrange all
the forms in a truly natural classification, which shall express the
various interrelations of the genera, yet several beautiful series have
already been determined. In these series the individual development
of the later general shows transitory stages which are permanent in
antecedent genera. To give a mere catalogue of names without figures
would not make these series more intelligible.

The Brachiopoda, or "lamp-shells," are a phylum of which comparatively
few survive to the present day; their shells have a superficial likeness
to those of the bivalved Mollusca, but are not homologous with the
latter, and the phylum is really very distinct from the molluscs. While
greatly reduced now, these animals were incredibly abundant throughout
the Palaeozoic era, great masses of limestone being often composed
almost exclusively of their shells, and their variety is in keeping with
their individual abundance. As in the case of the ammonites, the problem
is to arrange this great multitude of forms in an orderly array that
shall express the ramifications of the group according to a genetic
system. For many brachiopods, both recent and fossil, the individual
development, or ontogeny, has been worked out and has proved to be
of great assistance in the problems of classification and phylogeny.
Already v