ADAMS, PHYLOOENY OF THE JAW MUSCLES I37 



liar triangular plate on the outer face, and at the posterior end this 

 triangular plate is roughened for the insertion of a supporting cartilage 

 or ligament. Ver}- probably it articulated with the inside of the cheek 

 plate or with some other external plate and not with a quadrate. In the 

 mandible of Mylostoma Dr. Eastman (1906) found an irregular mass on 

 the inner side which he interpreted as the Meckelian cartilage. But if 

 these mandibles were articulated with a quadrate they should show some 

 indication of it at their posterior end. If the mandible, with the strong 

 action that must have been present, as evidenced from the great marks 

 of the shear, were pulled against the quadrate by the usual adductor 

 muscles of the Pisces, it is reasonable to suppose that instead of being 

 blade-like the arthrodiran mandible would show the characteristic round- 

 ing and blunting that comes with this kind of strain, as is shown in every 

 mandible of Pisces where there is strong pressure in the back part of the 

 mandible. 



The reconstruction of the musculature of Diniclitliys mandibles, as 

 shown in Plate XI, is worked out from a mechanical point of view, as it 

 appeared to be impossible to adapt the ordinary piscine musculature to 

 the arthrodiran jaws. In this reconstruction it is considered that there 

 must have been a synchronous movement of the skull and mandibles, and 

 the musculature is figured out on this basis. From the peculiar construc- 

 tion of the head, it is inferred that Dinichthys and its allies are the end 

 members of a group that moved the head and had the jaws more or less 

 fixed. The most efficient mechanical construction seemed to require that 

 the inner face of the mandible should be connected by ligament or muscle 

 with the "clavicular element," which extends forward on the inside of the 

 mandible. If the attachment at this place were by ligament and fascia 

 and the posterior end were appressed to the inner side of the cheek plate 

 or to some other plate in this region and worked as the scapula works on 

 the body of a mammal, we would have the movement desired, as the rais- 

 ing of the head would aid in raising the back part and lowering the front 

 part of the mandible, while lowering the head would close it. The mech- 

 anism of this part must have been very perfect, for the shear is always in 

 one plane, with no rounding of the edges. On the inner side of the 

 mandible at the anterior end are depressions, that show where the mandi- 

 bles were fastened by strong ligaments. 



In conclusion, the evidence gained from a study of the jaw mechanism 

 of Dinichthys seems to favor the conclusion which has been advocated by 

 many writers, especially Dean and Hussakof, that the Arthrodira are 

 related by common origin with the Antiarchi. 



