8 OSTEOLOGY OF PTERANODON. 



The largest opening seen in the side view of the skull was originally termed by 

 Professor Marsh " the antorbital vacuity." Professor Williston describes l and figures an 

 oval depression or excavation in front of the orbit in Nyctosaurus, which in his opinion 

 " clearly corresponds to the antorbital vacuity of the earlier pterodactyls." He con- 

 cludes : " From this it is evident that the antorbital vacuity is not united with the nares 

 in this, genus or Ornithostoma' 1 '' (Pteranodon). This depression does not appear in the 

 types of P. longiceps and P. ingens, and the term " antorbital vacuity " is here retained. 



The orbit as seen in the type is pear-shaped, with the elongation forward and down- 

 ward, in this respect as well as in its position above the infratemporal vacuity differing 

 greatly from the orbit of Nyctosaurus, in which genus it is situated much further for- 

 ward, overlying in fact the anterior end of the quadrate when the line of the jaws is 

 horizontal. 



The supraorbital ridge of the frontal projects boldly outward over the orbit and is one 

 of the stoutest parts of the skull. The orbits are without floor and open anteriorly into 

 each other and into the great narial cavity. The interorbital septum as seen in the 

 type rests upon the entire length of the basisphenoid, its anterior edge leading back- 

 ward and upward until on the level with the center of the orbit, when it curves sharply 

 forward and upward to merge with the under surface of the frontals. The sclerotic 

 circle has been lost in the skull of P. longiceps, but is excellently preserved in the type 

 of P. ingens (Plate III, figure 1, and Plate V, figure 3). 



Lower Jaws. 



The articular part of the mandible has been already described in connection with the 

 remarkable form of the quadrates. As seen in the lateral view of the type skull of 

 P. longiceps (Plate I, figure 1), the lower jaws fit closely against the cutting edges of 

 the maxillary arch, which they simulate in general form. In the type, the points of both 

 the upper and lower jaws are wanting, and no specimen in the Marsh Collection is 

 complete in this respect. By fairly carrying out the lines of the remaining portions of 

 the jaws, extremely slender terminal points are indicated. That this was the true form 

 of these parts in the smaller allied genus Nyctosaurus, is clearly shown by abundant 

 material, and the wonderful delicacy of other portions of the skeleton of Pteranodon 

 renders such a hypothesis tenable. 



For a little over two-thirds of their total length, the mandibular rami meet in a firm 

 symphysis. Behind this, the separated rami quickly diminish in depth and increase 

 slightly in transverse diameter, their form becoming oval in section immediately in front 

 of the quadratic articulations. Posterior to the articular facets there are small angular 

 projections. 



Professor Williston has given an excellent restored figure of the posterior half of the 

 mandible, seen from above (Note on the Mandible of Ornithostoma, Kansas Univ. Quart., 

 vol. iv, July 1, 1895). The Marsh Collection contains a fragment of this part, No. 2478, 

 which is here figured from the right and left sides (Plate I, figures 2 and 3), so as to 

 show the posterior part of the mandibular symphysis and the bony floor of the oral 

 cavity, which united the parapets of the jaw nearly as far back as the termination of 

 the symphysis. The front end of the fragment can not well be figured, but it demon- 

 strates the presence of a long cavity of triangular section, enclosed between the floor 



1 Ibid. 



