No. 444 ] PTERASPIDAZ AND CEPHALA SPIDAZ. 83 1 



tainly be a very unusual thing it" all the ostracoderms mimicked 

 animals so different in grade of organization according to Wood- 



icked the Mer..stommata. than that the Merostommata mimicked 



tage to any of them on either supposition. 



Moreover the features in which the ostracoderms mimicked 

 the eurypterids are characteristic of a very extensive class and 

 are the very characters which are important in differentiating the 

 ostracoderms from the true fishes, such as, for example, the 

 small pointed body, large shield-shaped head with its peculiar 

 cornua, cephalic appendages, shell covered orbits, unusual char- 

 acter of the parts surrounding the mouth, and the minute 

 structure of the nearly continuous dermal armor. It is the 

 combination of all these characters that makes the resemblance 

 between the ostracoderms and Merostommata difficult to under- 

 stand on any other assumption than that of genetic relationship. 



Clearly it is not convincing, or a final solution of the problem. 



classes of animals are due solely to either mimicry or parallelism. 



II. The PTERASPiDiE. Going back again to the older 

 writers, we shall see that much of their discussion having am 

 bearing on the position of the ostracoderms was on the struc- 

 ture and relations of the Pteraspkke. But the fact that these 

 animals were the first fish-like animals to appear on the earth's 



thought 1 

 bite-like . 



