INTRODUCTORY 



THE diagnoses given in this classification are not exhaustive 

 definitions, although often more than sufficient because of what 

 may be called additional characters. For instance, the " pos- 

 session of visceral arches, one pair of which is modified into 

 jaws," is a quite sufficient diagnosis of the Gnathostomata. 

 The presence of an anterior and a posterior pair of limbs is 

 probably quite as essential and peculiar a feature. There are 

 not, and can never have been, paired-limbed vertebrata without 

 visceral -arch jaws; consequently, wherever the converse is 

 the case, we feel certain that the absence of limbs is a 

 secondarily produced feature. This may serve as an example 

 of admitting certain fundamental characters which may not 

 be applicable to all the members of the group in question. 



Various features which we are accustomed to associate 

 with the description of the recent members of a class, order, 

 or family for instance, the intestinal spiral valve of Plagio- 

 stomi have not been mentioned ; partly on account of our 

 imperfect knowledge of the fossil forms, partly because these 

 features do not apply to such fossils which are undoubtedly 

 not only closely allied to, but ancestral to the same group in 

 question. On the other hand, it would be pedantic to exclude 

 all soft, perishable parts on the plea that they are unknown in 

 the fossil forms. Here discretion is to be used. We do not 



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