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Bashford Dean Memorial Volume 



most.enigmatic parts of the whole fish. From the earliest times investigators have wondered 

 about this remarkable jaw structure, which is without any analogy in other animals. 

 Miller, in "Old Red Sandstone" (1841) described the small teeth placed in the symphysial 

 part of the lower jaw. This discovery, later confirmed by other investigators (Pander, 

 Traquair, Woodward, Newberry, Dean, Hussakof), leads one to suppose that the lower 

 jaw halves in the Arthrodira were not connected in front. When explaining this char- 

 acter so unusual for vertebrates, the different authors proposed diverse theories. Miller 

 considered that the mouth in Coccosteus, like that of the 

 Arthropoda, was vertically, not horizontally, placed. In 

 his opinion the symphysial teeth worked against each 

 other. Nearly the same opinion was expressed by Patten 

 (1912). He said that: "The mandibles were capable of 

 very complex movements. Both ends were free and could 

 be either rotated, or moved in a transverse and longitu- 

 dinal direction." 



A very careful investigation of the movement of the 

 jaws in the Arthrodira — especially in Dimchthys — was 

 made by Hussakof (1906). In his opinion, the lower jaw 

 was not connected in symphysis. In opening the mouth 

 the lower jaw not only moved downwards but also side- 

 ways. Hussakof thought to find a confirmation to this 

 fact in marks of wear on the "teeth" and in bite marks, 

 which can sometimes be found on the armor plates of 

 Dinichthys. 



Contrary to the opinions of these three authors, 

 who considered the Arthrodire mouth built in a way 

 unusual to other vertebrates, other investigators have held that the Arthrodire lower jaw 

 was formed and that it functioned on the same principle as the jaws of other fishes. New- 

 berry was the first to compare the jaws of Dinichthys with the mouth apparatus of the 

 Dipnoi. According to him the rami of the lower jaw were connected in symphysis and 

 attached to the cranium by the help of cartilage. Traquair (1900), Jaekel (in a paper be- 

 fore 1907), more recently Woodward (1922), and Stensio (1925) supposed the mouth con- 

 struction in the Arthrodira to be of the usual fish type. 



Opposing these opinions stands the theory of Jaekel and of Adams. As early as 

 1907, in 'describing the morphology of Pholidosteus, Jaekel mentioned that in opening the 

 mouth the upper jaw (head) in the Arthrodira moved more than the lower jaw. In 1919 

 he gave a more detailed description of the jaw mechanism in the Arthrodira (Text-figure 

 75). According to him the hind corner of the lower jaw was by the aid of a special plate 

 ("Articulare" = PIG) attached to the post-sub-orbital plate ("Quadrato-jugale"). In open- 

 ing the mouth, the head roof by the contraction of the strongly developed muscles run- 

 ning from MB to MD was lifted up around the joint between the head and body. The 



Text-figure 74. 



A reconstruction of the ligament 



attachment of EB and ADL in the 



neck joint of Dinichthys. 



(Compare with Text-figure 73). 



Ig, ligament, rg, ndge supporting condyle. 



