DEVONIC FISHES OF THE NEW YORK FORMATIONS I I I 



natural association with the head shield, or even in the detached condition, 

 which can be interpreted with any degree of assurance as belonging to the 

 dermal covering of the trunk. Analogy with other Arthrodires is indeed 

 suggestive of the development of body armor ; but on the other hand regard 

 must be paid to the fact that no indications of overlap nor of an articular 

 joint are visible along any portion of the posterior margin, and that the 

 sensory canals stop short of it, whereas in other genera they are continued 

 across the cleft between head shield and body armor. Conclusions based 

 upon negative evidence are liable at any moment to be overturned by new 

 discoveries ; but in the present case this is the only class of evidence avail- 

 able, and the inference to be drawn from it is that the most primitive 

 Arthrodires resembled Ceratodonts in being unprotected by abdominal 

 armor. In all groups the development of an extensively ossified exoskeleton 

 is looked upon as evidence of specialization. Its nondevelopment amongst 

 forms that are clearly primitive in other respects would be, therefore, in 

 nowise startling. Moreover, it should be remembered that within a single 

 family of Ostracophores, Pterichthys is scaled, Bothriolepis naked. 



In view of the remarks that have just been offered it is scarcely neces- 

 sary to say that the view formerly entertained by the present writer, and 

 more fully developed by Dean in his definition of "Anarthrodira," accord- 

 ing to which all that portion of the head shield lying posterior to the chon- 

 drocranium corresponds to the dorsal body plates of typical forms, is now 

 rejected as erroneous. The median and external occipital plates in this 

 species are not superimposed upon a separate system of underlying plates, 

 and whatever notion may have existed to the contrary may be set down to 

 deceptive appearances. Thus, the plates called by Newberry " parietal," 

 "squamosal," and " epiotic," in the region covered by the external occipital, 

 have no real existence, the supposed sutures between them being merely 

 the impressions of oblique ridges and other markings of the single large 

 plate which forms the posterolateral border of the head shield. The disso- 

 ciated elements belonging to a single individual are shown in plate 9, 

 figure 5. 



