190 Bashford Dean Memorial Volume 



usually long, highly compressed neural spine, which in its form resembles the keel on the 

 MD of Dinichthys. To this spine were attached the muscles going from the back of the 

 head. They served to lift the head. When using the saber-teeth not only the mouth 

 opened, but also the whole head was lifted up with the help of the previously mentioned 

 muscles. Without doubt, the same happened in Dmichthys, the sharply pointed ASG 

 was used as a very effective weapon comparable with the saber-tooth of Smilodon. The 

 greatest difference in the arrangement of the muscles and their attachment in these two 

 cases is that in Dinichthys they were attached to the dermal skeleton (head roof and MD), 

 in Synilodon to the inner skeleton. Thus, in the first, the keel extended from the surface 

 into the body; in the second, from the axial skeleton out to the surface. But the function 

 in both cases was the same. 



The second strong pair of muscles, which can be called "depressores capitis" (DC, 

 Adams: Text-figures 76 DC, 79-81, 83 and 84 II) serve to move the head downwards 

 and are, of course, antagonistic to the levatores capitis. Jaekel held that no muscles were 

 necessary to move the head downward, that the heavy head would fall down on the lower 

 jaw by itself ("Fallbiss"; Jaekel, 1919). This theory is quite improbable. Though m 

 Dinichthys, perhaps, the head was heavy enough to fall down with relatively great power, 

 in all smaller forms (Coccosteus) this would not happen. The resistance of tissue and of 

 water, would make the "fall down" without help of muscles too slow and powerless. 



Adams attached these muscles with one end to the side margin of the head (along the 

 marginal) where there is a long ridge (Text-figure 13, LCP, Rd), and with the other end 

 to the middle part of AL. This position of the muscles is, however, hardly acceptable. 

 SO is in contact with the ridge along the marginal and there is no place to attach any 

 muscles here. At the same time, thus attached the depressor capitis on each side would 

 cover the opening between PSO and AL. Here, as we shall see later, were the gill 

 openings. 



In my opinion, the musculi depressores capitis were fastened higher up than proposed 

 by Adams. In the head roof they occupied the deep impression (Text-figures 13 PL, 79, 

 80, 83 and 84 11) on both sides of the median-basal and were attached to its bottom and 

 steep walls. In the body carapace they were attached to the upper part of the high ridge 

 running on the underside of AL (Text-figures 51, 79 and 80). Each muscle was com- 

 paratively short but broad, and was very strong. It performed the same work as the mus- 

 culus adductor mandibuli (Adams) in other fishes. Like the last it was a bite and chew 

 muscle, but it worked in the opposite direction to the adductor mandibuli, which moved 

 the lower jaw up, while the depressor capitis moved the upper jaw down. Being 



very strong, this muscle with its position so near the joint gave the upper jaw a special 

 power. The teeth of the upper jaw, placed far from the joint, were moved downwards 

 with a horrible power. Since Newberry's time marks of teeth have been known on some 

 body plates of other specimens. Some plates with undoubted marks are preserved in 

 the American Museum. One described by Hussakof (1906) is especially interesting since 

 it has deep round cuts like that from a pair of teeth. Hussakof thought that these cuts 



