I?. S. Lull — Cranial Musculature in Dinosaurs. 389 



their origin on the dorsal surface of the parietals, one on each 

 side of the median ridge of the frill and passing directly down- 

 ward and forward to be inserted into the posterior margin of the 

 coronoid process (Plate I). The direction of pull is met by 

 that of the quadrate, whose oblique position is such as to with- 

 stand the strain to the best advantage. The high coronoid 

 gives a long power arm which, though at an angle of 90° 

 with the axis of the jaw, affords admirable leverage for the 

 masticatory movements. The pterygoid muscles, both external 

 and internal, must have been present ; their direction of pull 

 was more nearly vertical, and, while not strongly developed, 

 they undoubtedly seconded the temporal muscle to a certain 

 extent and guarded against dislocation. 



A masseter muscle may also have been present having its 

 insertion along the coronoid process just outside of that of the 

 temporal and arising from the forward margin of the jugal 

 bone. 



The jaw was opened by means of the depressor mandibular, 

 having its origin on the posterior and inner face of the quad- 

 rate, possibly to a certain extent on its outer face, as well as 

 upon that of the quadrato jugal. This was a rather broad, sheet- 

 like muscle of moderate thickness and was inserted, as indicated 

 above, into the lower aspect of the articular, angular, and 

 splenial, possibly to a slight extent on the surangular. Cheek 

 muscles must have existed, originating on the outer side of the 

 maxillary and the posterior portion of the premaxillary as 

 indicated by a sudden inward compression of the lower portion 

 of these bones along a line running obliquely downward and 

 forward from the jugal to the lower margin of the premaxillary 

 bone. The insertion of this muscle lies along the forward 

 margin of the coronoid and sweeps forward along the outer 

 surface of the jaw, finally rising again to the end at the upper 

 termination of the dentary-predentary suture. This broad 

 sheet of muscle, probably equivalent to the buccinator, was 

 subsidiary to mastication, as its chief function was to retain 

 the food in the mouth. The extent of this muscle limits the 

 backward extent of the gape of the mouth, as the writer 

 (Lull, 1905) has shown in previous papers. 



Muscles of the Neck. 



The occipital condyle is hemispherical and fits into a corre- 

 spondingly deep depression in the atlas. The extent of the 

 two articular surfaces is so nearly equal that a very slight 

 movement of the cervicals causes one facet to go past the limits 

 of the other, as verified by actual experiment. This would 

 seem to imply a very limited range of movement at this point, 

 the hemispherical condyle being an ancestral feature retained 



