PHYLOGENESIS IN CLASSIFICATION. 239 



meaning of the distinctions pointed out in these pages is to take a lot of 

 mollusca of different kinds, to be found on the sea-shore, and with the de- 

 scriptions of the expert zoologist at hand attempt to identify and classify 

 them. 



And the naturalist who may possibly look into these pages will appre- 

 ciate, no more keenly than the author, the great difference between such 

 an introduction as is here attempted, and the earnest investigation of the 

 history of organisms by a study of the organisms themselves. 



Mollusca and Brachiopods as Illustrations of Evolutional History. 

 The Mollusca and the Brachiopods present a peculiar inter- 

 est, because, having no skeletal parts, the mode of action, 

 the result of adjustment to environment, the adjustment of 

 several parts and organs to each other in body structure, and 

 the marks of stages of growth are all concentrated in an ex- 

 ternal, hardened, and therefore preserved, single, or rarely 

 more than two-parted shell. 



To the extent to which such a shell can express the char- 

 acters of the organism, the perfection of its preservation, and 

 the fact that so much of the individuality of the species and 

 so large a number of individuals are accessible to study, give 

 to this kind of fossil its great value in illustrating the problems 

 of evolutional history. 



Zittel's Classification of the Branch Mollusca,* The classifica- 

 tion of the Mollusca proposed by Zittel differs in some re- 

 spects from that of Lankester, Gegenbaur, and many of the 

 stricter modern zoologists. 



In his branch Mollusca were included as sub-branches 

 A. Molluscoidea (which is a branch in Gegenbaur's 

 classification), 

 with the Classes I. Bryozoa 



II. Tunicata (which is raised to 

 the rank of a distinct branch 

 by Gegenbaur); 

 and III. Brackiopoda. 

 (Gegenbaur places Bryozoa with the Worms 



* Note: This classification was taken from the " Handbuch," published 

 nearly twenty years ago. In the author's " Grundziige " (1895), the Mol- 

 lusca and Molluscoidea are relegated to separate branches, in accordance 

 with present usage. The above passage is left as originally written 

 because it well illustrates the point under discussion. See Am. Jour. Sci. y 

 ser. HI, vol. L, p. 268. 



