The Structure of Dinichthys 187 



jaw, however, is more changed. Its outer surface becomes strongly bent inward. This 

 makes the lower front corners of the left and right IG, come closer together (Text-figure 

 78). They were probably connected by a hitherto unknown bone, or by cartilage and 

 Hgaments. (It may be pointed out that the inner surface of the front part of IG is rough 

 which indicates the presence of cartilage). At the same time the points of the big "front 

 teeth" are extended quite far apart from each other (Text-figure 78). This explains very 

 satisfactorily the presence of the symphysial teeth. Only the lower front corners of both 

 IG plates were connected. The upper parts, where the symphysial teeth were placed, 

 have never touched each other. 



Surely all these facts indicate that the lower jaws of the Arthrodira were connected 

 in symphysis and in an extremely unusual manner. The drawing in Text-figure 78 shows 

 the lower "jaw" of Dinichthys from the front. The new arrangement of IG makes the 

 whole jaw structure much more soHd and compact. 



THE MECHANISM OF THE MOUTH MOVEMENT 



Among the theories mentioned in the last section, those of Miller, Hussakof, and 

 Patten must be excluded as incompatible with the newly known facts. The opinion of 

 these three authors was that IG in the Arthrodira was not articulated to the cranium and 

 not fused in the symphysis. But we know that the lower jaw was connected to PSO, 

 and, in all probability, also fused in symphysis. 



The theory, that in the Arthrodira the mouth was built on the same principle as 

 that of other fishes (Traquair, Newberry, Woodward, Eastman, Stensio), is also more or 

 less unacceptable. The joint between the head and lower jaw in the Arthrodira, as 

 we have seen in a previous section is very unsatisfactorily developed. In fact, it is not a 

 joint, merely an attachment point. In a fish like Dimchthys, with very strong, heavy teeth, 

 such a weak joint would exclude the possibility of effectively working jaws. Also, the 

 thin blade of the lower jaw is not solid enough to serve as an attachment for the strong 

 muscles necessary for moving the jaws up and down. 



The theory proposed by Jaekel and by Adams is the only one giving a logical and 

 satisfactory explanation of all the remarkable facts in the structure of the Arthrodira 

 and a complete picture of the mechanism of the jaws — an unexpected mechanism, never 

 before observed in any other animal living or fossil. 



The following facts may confirm the correctness of this Jaekel- Adams theory, and 

 the more we study the structure of Dinichthys in detail, the more probable this theory 

 becomes. These facts are: 



1 . The presence of a joint between the head and body allows the head to move up 

 and down. This joint, very strongly developed in all Arthrodira and Antiarcha, must be 

 of vital significance for these animals. Nearly all living and fossil fishes have the head 

 practically immovably connected to the body. It is difficult to understand why it was so 

 necessary for the Arthrodira to move the head up and down. Whereas in typical benthonic 

 forms, such as the Acanthaspida, Homosteida or Antiarcha, this arrangement is intelligible 



