198 The Palaeontological 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," 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 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 



