The Structure of Dinichthys 



183 



were covered with articular cartilage. The 

 whole joint was also safely protected and 

 bound together by ligaments. We can very 

 clearly see a trace of ligament'attachment on 

 both ADL and EB. Especially on ADL it 

 is easily found. Beginning on the extreme 

 margin of the processus sub-glenoidalis, it runs 

 along the edge of the supporting ridge to its 

 point, thence back again on the other side of 

 the ridge and around the upper joint fossa 

 to the margin of ADL. It continues along 

 the margin making a very sharp line on the 

 front part of the condyle, and at last comes 

 back to the margin of the processus sub- 

 glenoidalis. On EB the ligament was attached 

 around the fossa and along the edge of the 

 processus glenoidalis and the upper joint pro- 

 cess (Text-figure 74). 



It is obvious that such a complicated and 



Text-figure 73. 

 The neck joint in Dinichthys shown in surface view 



and in section, 

 eg, condyle; d, cleft between processus glenoidalis and hind margin 

 ofEB, l;p, lower joint process; pg, processus glenoidalis; psg, pro- 

 cessus sub-glenoidalis; rg, supporting ridge for condyle; uj/, upper 

 joint fossa; ujp, upper joint process. 



ADL 



Text-figure 72. 

 The joint process on the front margin of the antero- 

 dorso-lateral plate : A, from above; B, from below. 



a-b, axis of condyle; eg, condyle; psg, processus sub-glenoid- 

 alis; Tg, supporting ridge for condyle;' ujf, upper joint fossa. 



solidly built joint must have been of a vital 

 importance to the fish. It allowed the 

 movement of the head in relation to the 

 body and thus is analogous " with the oc- 

 cipital joint between the cranium and 

 spine in other vertebrates. 



THE RELATION BETWEEN THE 

 GNATHAL ELEMENTS 



Before we can describe the mechanism 

 of the movement of the head and jaws in 

 Dinichthys and the importance of this to 

 the animal, it is necessary to describe in 

 detail the relative position of the gnathal 

 elements and to give the different theo- 

 ries about the jaw mechanism in the 

 Arthrodira. 



The jaw elements in the Arthrodira 

 are the most unsatisfactorily known and 



" It is obvious that this joint can not be regarded as homologous with the occipital joint. It is placed on the side of the head roof, 

 far from the occipital region of the head and from the spine. 



