198 The Palacontological Record. I. Animals 
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,” ie. 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 genera 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 com- 
paratively 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 
very encouraging progress has been made in the solution of these 
problems. All brachiopods form first a tiny, embryonic shell, called 
the protegulum, which is believed to represent the ancestral form of 
the whole group, and in the more advanced genera the developmental 
stages clearly indicate the ancestral genera of the series, the suc- 
cession of adult forms in time corresponding to the order of the 
ontogenetic stages. The transformation of the delicate calcareous 
supports of the arms, often exquisitely preserved, are extremely 
interesting. Many of the Palaeozoic genera had these supports 
coiled like a pair of spiral springs, and it has been shown that these 
genera were derived from types in which the supports were simply 
shelly loops. 
The long extinct class of crustacea known as the Trilobites 
