208 Bashford Dean Memorial Volume 



2. As seen, quite likely the post-infero'gnathal in many forms was cartilaginous. 

 Possibly even in Dinichthys it was only superficially ossified. PIG may thus not have 

 been a dermal bone, and we must suppose the same about IG, as PIG was placed on the 

 outside of the latter. 



3. In Acanthaspida, one of the oldest known Arthrodires, the jaw elements have 

 never been found. Also in a previously mentioned specimen from Germany (Text' 

 figure 87) showmg a perfect preservation, no traces of them are seen. This indicates 

 that the jaws of these oldest forms were perhaps made of cartilage. 



4. The head shield of the Arthrodira is naturally divided into two parts; the head 

 roof, and the side plates of the head. This division reminds us much of that in other 

 fishes, where we find corresponding parts in the neurocranium and the visceral skeleton. 

 These elements are perhaps homologous. 



5. The sensory canal system of the Arthrodira is not so unlike that of other fishes. 

 The homology is not clear, but still acceptable. 



These are the facts we know. They are not of an indisputable nature, and the 

 question about the relationship of the Arthrodira will in the end be settled by the 

 personal judgment oi' the investigator. For myself, I hold to the opinion that the more 

 completely the Arthrodira have been studied, the more striking appears their dissimilarity 

 to all other types of fishes. Many characters show such fundamental differences that it is 

 nearly impossible to correlate the Arthrodires with other fishes. In all cases, our knowl' 

 edge of today yields no facts in proof of a sure relationship between them and other known 

 groups of animals. 



THE RELATION OF THE ARTHRODIRA TO THE ANTIARCHA (ASTEROLEPIDA) 

 The older investigators (Agassiz, Miller, Pander, Huxley, Eichwald, Zittel) have 

 always placed the Arthrodira in close relation to the Antiarcha. But Newberry, who 

 thought he could show a relation between the Arthrodira and the Dipnoi, detached the 

 two groups mentioned above and placed them far from each other in the system. However 

 as early as the beginning of this century many authors pointed out the great likeness between 

 these groups and suggested their possible relationship (Jaekel, Dean, Hussakof, Patten, 

 and more recently Stensio and Stetson). 



As a matter of fact, if the structure of the head and body armor in Arthrodira and 

 Antiarcha can not be homologized in detail they show, nevertheless, similarities in their 

 larger structural characters. These may be noted as follows: 



1 . They have both the head and forepart of the body covered by armor. 



2. This armor is divided into head and body portions. 



3. The head and the body armor are movably connected with the help of a double 

 neck joint, a character unknown in other animals. 



4. The armor is built of more or less isolated bony plates which are connected by 

 squamosal sutures. 



5. The eyes are protected by sclerotic rings. 



