The Structure of Dinichthys 



153 



of SO and PJ^ was, in reality, strongly 

 curved inward. In the drawing they are 

 pictured in a plane in order to show the 

 dental elements more clearly. The same 

 thing applies to the front part of IG. The 

 position of the upper dental elements 

 (ASG and PSG) can not be regarded as 

 absolutely determined. They were cer- 

 tainly connected to SO with ligaments 

 or cartilage. This makes it very difficult 

 to fix their position. But the proposed 

 relation is quite probable. 



In another section I shall discuss the 

 movement of the lower jaw and how the 

 "teeth" worked against each other. 



RECONSTRUCTION OF THE HEAD 

 SHIELD OF DmiCHTHTS 



The greatest difficulty in the recon- 

 struction of the head roof of Dinichthys is 

 to find its correct curves from side to side 

 and from back to front. Older investi' 

 gators saw clearly that the head roofs we 

 find in the field have been without ex- 

 ception greatly flattened and that they 



were in reality much curved. Thus, the question is: How can we distinctly determine 

 the degree of this bending? Fortunately we find some facts in the head structure which 

 give us a very exact answer to this question. 



As we know, the head roof of the Arthrodira was movably articulated with the help 

 of a joint to the body carapace. Two oblong articulation sockets are placed on EB, on 

 the hind margin of the head. Two oblong condyles are to be found on the antero-dorso- 

 lateral plates on the body carapace. Surely the movements of the head were only possible 

 if the long axes of both sockets (and of course also of both condyles) were placed strongly 

 horizontally. Moreover, the axes of the right and left sockets lie in a continuous hori- 

 zontal line, the motion axis of the head. If the sockets are not placed as mentioned above, 

 their axes do not form a continuous horizontal line. The movement of the head is then 

 absolutely impossible, one socket would hinder the movement possibility of the other. 

 We can find an analogy in the jaw-articulation of higher vertebrates. 



No attention has ever been paid to this matter, and it has been overlooked that the 

 possibility of finding the real curve of the hind-part of the head roof is offered here. 

 Professor Jaekel (1925.2) noted the correct position of the joints in the Arthrodira, with- 



Text-figure 34. 



Reconstruction ot the side plates of the head of 



Dinichthys: A, from outside; B, from inside. 



ASG, antero-supra-gnathal; IG, infero-gnathal; PIG, post-infero- 

 gnathal; PA(, post-nasal; PSG, post-supra-gnathal; PSO, post-sub- 

 orbital; SO, sub-orbital. 



