5 Introduction 



Several recent schemes of classification in particular groups are 

 known to the compiler but are not followed herein. Some were received 

 too late for study (e.g., part W of the Treatise of Invertebrate Paleon- 

 tology). Some were not yet available in the form needed for our use 

 and so were not considered (e.g., Echinodermata by H. B. Fell and 

 Mollusca by Taylor and Sohl). There is no judgment of these schemes 

 implied in this action; they will be considered for a subsequent revised 

 edition. 



It will probably be thought by some that this is an extreme classifi- 

 cation in separating many small groups as distinct phyla. The compiler 

 believes that it is a conservative classification even in this regard. He 

 believes that an important basic tenet of classification, too often over- 

 looked, is that all groups must be distinct and definable and that there- 

 fore forms are not to be forced into existing groups at any level if 

 they do not agree with what are deemed to be the important features of 

 that group. The important features in this case are those which caused 

 the group to be set aside and maintained as distinct. 



It is sometimes possible to enlarge slightly the scope of a group 

 definition to admit forms previously unknown, but this does not justify 

 including widely divergent forms that cannot be defined together effec- 

 tively. 



