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THE AMERICAN NATURALIST. [Vol. XXXVII. 



mals are among the oldest representatives of their respective 

 types known, that they were contemporaneous and lived under 

 very similar conditions, and that they presented many obvious 

 external resemblances in form and mode of life, clearly invited 

 a more detailed comparison between them. 



The evidence thus far obtained, justifies the belief that pale- 

 ontology will add its evidence to that of anatomy and embryology 

 in favor of the origin of vertebrates from arachnids. 



From the following historical review it will be seen that while 

 it is now generally assumed that the ostracoderms have pro- 

 nounced affinities with the vertebrates, there is the greatest 

 diversity of opinion among leading authorities as to whether cer- 

 tain organs characteristic of the true vertebrates are present in 

 the ostracoderms or not, and there is also the greatest difference 

 of opinion in the interpretation of those structures which are 

 actually known to occur there. In many cases the despairing 

 admission is made, that if certain openings, or structures, are 

 not this, that, or the other, what can they be? This is not due 

 entirely to the imperfect preservation of the remains, because in 

 some cases they are beautifully preserved. 



At the very outset, the obvious differences between the 

 ostracoderms and true vertebrates, and the resemblance between 

 them and the arthropods, was a subject tor repeated comment. 

 In fact the resemblance was so striking that it led many of the 

 best earlier observers to describe certain merostomatous arthro- 

 pods as fishes and various ostracoderms as arthropods. More 

 recent authors while admitting that there is a superficial resem- 

 blance between these two great groups, explain this resemblance 

 as one due merely to mimicry or parallelism, and not to a genetic 

 relationship. Some authors even deny that the peculiar struc- 

 ture of the ostracoderms is an indication of their primitive 

 character, but regard them as either highly specialized, or as 



certain genera of the ostracoderms, especially 01 Cyathaspis and 

 these genera, and consequently the whole group of the ostraco- 



