The Structure of Dinichthys 209 



6. The body armor of the oldest known Arthrodira greatly resembles that of the 

 Antiarcha. 



7. The armor plates and jaws are built only of true bone. Real teeth are unknown. 



8. The gnathal elements in both groups seem not to be homologous with the jaw 

 elements of other animals. 



All these points permit us to accept a relationship between the Arthrodira and the 

 Antiarcha. However, we must remember that it is not a very close relationship and that 

 these two groups can not be regarded as forms derived from each other. 



THE SYSTEMATIC POSITION AND DIVISIONS OF THE ARTHRODIRA 



We have now seen that the Arthrodira can hardly be connected with any known 

 fish group. Further, their structure shows so many exceptional characters that in my 

 opinion we have not enough facts to consider them as belonging to the true fishes at all. 

 Therefore, with Dean and with Hussakof, I propose to establish an independent class of 

 animals which shall rank with Cyclostomes and Pisces. This class may be divided into 

 sub-classes — Antiarcha and Arthrodira. In my opinion it is more natural to retain the old 

 name Placodermata proposed for the same two groups by M'Coy (1848), than to adopt the 

 new name Arthrognathi used by Dean (1900). I give the following definitions of this 

 class and its sub-classes. 



Class PLACODERMATA 



Water-living, fish-like animals, with armored head and forepart of the body. The 

 head armor is movably connected to the body armor by means of a double neck joint. The 

 armor is composed of more or less separate bony plates, usually sculptured on the surface. 

 The sensory canals, in the form of deep grooves on the surface of the plates, are always 

 present. The eyes are protected by sclerotic rings. The opening (or deep impression) for 

 the pineal organ and that for a double nasal opening are well developed. The gnathal 

 elements are not homologous with the jaw elements of other vertebrates. No real teeth 

 are developed. The hinder part of the body is naked or covered with scales. The dorsal fin 

 is always present . The neurocranium, and other parts of the inner skeleton are never ossified . 



The Placodermata are found only in strata of the Devonian Era. 



Sub-class ANTIARCHA 



The condyle of the neck joint is on the head; the fossa condyli on the body. The 

 head is relatively small. The eyes are placed close together near the median line; the 

 pineal organ is between them; the double nasal opening is in front. The gnathal elements 

 are represented by two pairs of separate bones. The body armor is relatively long and 

 narrow. In all forms it is made up of the same number of plates, arranged after the same 

 plan. Two long, jointed appendages are movably connected to the anterior plate of the 

 ventral carapace by means of a remarkable joint. No traces of paired fins are present. 



The Antiarcha are found only in Middle and Upper Devonian. 



