The Structure of Dinichthys 



191 



were made by the front "pick" of the 

 lower jaw. I think differently. In the 

 first place, the lower jaw was not so strong' 

 ly muscled, and in the second, the marks 

 are on the MD plate. It is difficult to 

 imagine that the lower jaw could make 

 impressions on a dorsal plate, since in this 

 case one of the fishes, must have been 

 ventral side up. It is more likely that the 

 marks were made with the upper "teeth.'' 



The two other pairs of muscles are 

 more difficult to determine. They moved 

 the lower jaw up and down. As we know, 

 in raising the head, the front part of the 

 lower jaw automatically went down, as its 

 hind corner attached to PSO was lifted up. 

 The muscles now to be described merely 

 acted as a support in making this mechan- 

 ism more effective. 



The muscles of the first pair, which 

 we shall call "musculi levatores gnathalis" 

 were probably attached to the lower mar- 

 gin of IG with one end, and to the ridge 

 on the lower margin of SO with the other (Text-figures 79, 80 and 81 III). Of course, no 

 especially marked traces of muscle attachment are found either on IG or on SO. We must 

 suppose that such a muscle covered IG from the outside. The function of these muscles was 

 to stop IG in its downward movement, and in closing the mouth to move IG up to meet 

 the upper jaw pressed down by the depressores capitis. The muscles antagonistic to the 

 levatores gnathalis were the depressores gnathalis (Text-figures 79, 80, 81 and 89 IV). 

 Attached at one end to the underside of the front part of IG, where we find distinct traces 

 of muscles, they probably ran downward to AMVon the ventral shield. 



Stetson in his last paper, in discussing jaw muscles in Dinichthys, states that perhaps 

 one of the lower jaw muscles was attached to a process in the fork formed by two arms of 

 AL (Text-figures 50 and 52 th). I think it improbable, since muscles thus placed would 

 cover the gill openings. It is Hkely also that the muscles of the gill apparatus were partly 

 attached to the underside of IG. 



The musculi levatores capitis and depressores gnathalis worked together to open the 

 mouth. In contrast, by the contraction of the musculi depressores capitis and levatores 

 gnathalis the mouth was closed. These four pairs of muscles were of the greatest im- 

 portance in the mechanism of the mouth. Surely many others, smaller and weaker, were 

 attached to different parts of the upper and lower jaws, and helped in their work. 



Text-figure 81. 



Front view of the reconstructed head armor of 



Dinichthys — with the mouth open. 

 Ill, musculus levator gnathalis; IV, musculus depressor gnathalis. 



