390 i?. S. Lull — Cranial Musculature in Dinosaurs. 



in order to allow a rotary motion of the skull, as the usual 

 mechanism for permitting this movement, the articulation 

 between the atlas and axis, by means of an odontoid process, is 

 wanting, the first four cervicals being immovably coossified. 

 The chief movement of the head seems to have been accom- 

 plished by the bending of the neck as a whole. The normal 

 posture of the head was depressed, the muzzle coming rather 

 near the ground. The proof of this lies in the fact that the 

 condyle is borne on a stalk or peduncle which is bent down- 

 ward at an angle with the longitudinal axis of the skull. If 

 the cervical vertebrae are not in line with the axis of the con- 

 d} r le, it not only produces what engineers call an "invert joint", 

 an extremely weak structure mechanically, but the articular 

 facets no longer coincide. Plate II serves to illustrate this 

 matter. 



The ring-like atlas bears no transverse or other processes, 

 which, together with the fact that movement of the anterior 

 cervicals is so limited, makes it probable that some of the 

 muscles which are ordinarily inserted into the atlas have been 

 transferred to the occipital region of the skull. The rear of 

 the skull and the inferior surface of the frill exhibit a number 

 of well-developed muscle depressions which may be interpreted 

 as follows : 



The basioccipital shows, on either side of the median line 

 beneath, a deep depression, the insertion of the rectus capitis 

 anticus longus (rl) muscle, which has its origin on the ven- 

 tral aspect of the cervical vertebra? from the axis backward 

 (Plate II). This doubtless served to depress the skull and was 

 a muscle of moderate power. Smaller depressions, also in the 

 basioccipital, lying without the first mentioned, seem to have 

 been the insertions of the rectus capitis anticus brevis (rbj, 

 the origin of which was on either side of the axis. This was 

 also a depressor muscle of the skull as well as one which 

 swayed it from side to side but was relatively feeble, being in 

 all probability passively resistant rather than active in its 

 function. On either side of the foramen magnum may 

 be found small depressions evidently for the insertion of the 

 rectus capitis posticus minor (rmij, which in the turtle arise 

 from the neutral arch and diapophyses of the atlas. In Tri- 

 ceratops the point of origin was probably shifted backward to 

 the axis. 



Above, the supraoccipitals show large depressions into which 

 were inserted the rectus capitis posticus major (rma) muscles, 

 which arose from the neural spine of the axis. These were 

 muscles of considerable volume and aided in raising, or rather in 

 supporting the head. Thus there seem to have been, as in the 

 turtle, four pairs of muscles running from the anterior cervicals 



