388 Ji. S. Lull — Cranial Musculature in Dinosaurs. 



which bore the lower beak. The teetli form an admirable 

 chopping mechanism, shearing vertically past those of the upper 

 jaw. The worn face of the entire tooth series, however, is 

 not a perfect plane but is slightly twisted, being somewhat 

 oblique at the posterior and becoming vertical at the anterior 

 end of the row. The worn faces of the individual teeth 

 exhibit in some instances tiny oblique stride passing upward 

 and backward across the enamel in those of the lower jaw. 

 These would seem to indicate the direction of wear. The 

 articulation of the jaw is such as to permit some freedom of 



ptm. 



Fig. 1. Under side, rear, of lower jaw of Triceratops. 



motion, and, judging from these facts, it would seem as though 

 the jaw movement was in the main vertical, with a slight 

 lateral motion either to the right or left at the beginning of 

 the upward movement. When the final closure is reached the 

 jaws would, however, be in perfect alignment, enabling the 

 upper and lower beaks to form a perfect, turtle- like shear. 

 There may have been a slight movement to the rear, which 

 would account for the oblique strise on the teeth ; a forward 

 movement would of course be out of the question on closure, 

 as it would cause the wedge-shaped mechanism to bind. 



Muscle insertions are indicated on the jaw, first on the rear 

 margin of the very high coronoid process for the temporal 

 muscle; on the inner lower margin of the splenial and dentary 

 for the pterygoid muscles and on the lower face of the 

 articular, angular, and splenial for the digrastric or depressor 

 mandibuli (fig. 1). A passage on either side leads upward 

 and backward within the bones of the cheek, above the quad- 

 rate and exoccipital, and finally opens upon the upper surface 

 of the frill through the supratemporal fossa. These are 

 evidently the tracts wherein lay the temporal muscles, having 



