The Structure of Dxmchthys 



185 



hind corner of the lower jaw was thus also elevated, the symphysial part moved down, and 

 the mouth opened. In shutting the mouth, no strong muscles were necessary; the dorsal 

 muscles merely slackened and the heavy head fell down on the lower jaw f 'Fallbiss''). 

 Unfortunately Jaekel incorrectly included in the structure of the lower jaw a part of the 

 ventral carapace — the intero-lateral (his ''Angulare"). According to him this plate was 

 placed on the underside of the hind part of the "Spleniale" and when the mouth was 

 opened, it slid along the front margin of AVL. This arrangement is perfectly impossible. 

 When the mouth was opened, the lower jaw — as also the "Angulare" — moved up and 

 down in a plane almost parallel to the long axis of the body. The front margin of AVL 

 on the contrary is placed nearly perpendicularly to this plane, which excludes any pos- 

 sibility that the "Angulare" could slide along AVL (Text-figure 75). 



During the same year, Adams, independently of Jaekel, arrived at almost the same 

 conclusion in studying the jaws of Dmichthys (Text-figure 76). In his opinion the "man- 

 dible" was attached to the hind lower corner of SO. Otherwise he explained the mouth 

 mechanism somewhat in the same way as Jaekel. The only difference is that he also sug- 

 gested a pair of muscles for shutting the mouth. They were attached on one side to the 

 side margin of the head, on the other side to the lower part of the ''clavicular.'' 



This Jaekel-Adams theory was not generally accepted. Especially in Germany, 

 Jaekel's new theory was received with great scepticism. Later I shall try to demonstrate 

 its correctness. First, however, the relative position of the jaw elements must be studied. 



On all the gnathal elements in Dmichthys we find a strongly marked, polished cutting 



Text-figure 75. 



Jaekel's reconstruction of the jaw mechanism in the 



Arthrodira (1919). 



Text-figure 76. 



(R.glit) 



Adam's reconstruction of the jaw mechanism in 



Dmichthys (1919). 



DC, musculus depressor capitis; LC, musculus levator capitis. 



