Apr. 1903. North American Plesiosalrs — Willis ion. 17 



formed by the " frontals " is stout and rounded, and is continued at 

 least as far forward as the anterior end of the orbit, clearly separated 

 above and below by the median suture. The anterior ends are lost in 

 front in the broken fragments of bone, between and beyond the 

 anterior end of the orbits. Lying between the orbits, and separated 

 from each other by a narrow interval, are the narrow bones which may 

 represent the conjoined postfrontals and prefrontals and nasals. On 

 the right side, as stated above, the bone had been macerated away, 

 and while some of its processes had been broken off and lost, the 

 sutures for union with the parietal, frontal and prefrontal are beauti- 

 fully preserved, showing the relation to these bones in a way that 

 precludes doubt. The bone shows no trace of division whatever into 

 its supposed elements. It articulates with the "frontal," parietal, 

 epipterygoid, " postorbital," '• supraorbital," 'premaxilla and maxilla. 

 Posteriorly the bone extends downward, outward and backward to the 

 upper margin of the epipterygoid protuberance; externally and 

 posteriorly it sends off a projection for union with the post-orbital ; 

 anteriorly the bone fits into a groove on the outer side of the facial 

 processes of the premaxillaries for a distance of thirty or forty milli- 

 meters, and has a stout process on the outer side for union with the 

 supraorbital, or whatever the element may be here. On the under 

 side there is a broad, flattened, vertical plate, continuous from the 

 posterior, inferior angle, and widened in the middle so as to reach the 

 greater part of the way to the upper surface of the palatal bone, form- 

 ing the inner wall of the orbit in large part. The plate given off for 

 union with the "supraorbital" is separated by a sharp, deep notch 

 from a similar process for union with the " postorbital. " The ' ' supra- 

 orbital" bone has been crushed back over this process, so that the 

 distinguishing suture can be perceived in one place only, anteriorly. 

 In front of the orbit, the bone sends out a thin, triangular plate, 

 which curves downward to meet the maxillae, separated from its mate 

 by the premaxillae. Doubtless this part represents the nasal, and per- 

 haps also the lachrymal, but there are no indications of distinguishing 

 sutures, and I do not believe that the nasal exists as a separate ele- 

 ment in the adult plesiosaur — I can not find that it has ever been 

 described in any plesiosaur. It joins the maxillae broadly and the 

 "supraorbital" behind; in the angle between the three bones is 

 located the small external nares. Below the supraciliar plate, near 

 the anterior part of the orbit, on the side of the prefrontal, there is a 

 well-defined fossa, leading forward into the ethmoidal region, into 

 which opens a small foramen from the upper surface between the pre- 

 frontal and supraorbital. 





