i8 Field Columbian Museum — Geology, Vol. II. 



The supraorbital forms, as already stated, a horizontal plate 

 extending out over the orbit in front. Its union with the prefrontal 

 posteriorly is obscured by fracture, but indications of a suture are 

 seen anteriorly. Between this bone and the postorbital there is a 

 deep notch, angulated externally. The suture between the prefrontal 

 and supraorbital is clearly seen anteriorly, running from the small 

 foramen already mentioned forward and outward to terminate near 

 the maxilla, at the posterior end of the nares. The connection of the 

 bone with the maxilla can not be made out, as there has been an 

 infolding here ; its connecting suture with the ascending process of 

 the maxilla is, however, well defined, running obliquely forward. 

 The descending plate of the supraorbital has, in its orbital margin 

 near the upper part, a small foramen, piercing the bone obliquely. 

 The horizontal portion terminates anteriorly by sinking to the surface 

 of the descending portion. The whole bone reminds one of the pre- 

 frontal of Clidastes, 



The postorbital bone is a narrow, elongate and thin bone, united 

 above with the postfrontal, and to a slight extent with the parietal, 

 near the top of the epipterygoid ; below to the jugal. On the right 

 side, this bone, like the postprefrontal and jugal, has been macerated 

 away, and, although somewhat distorted, presents no evidence of 

 being composed of more than one element. 



In the above description of these frontal elements, I have fol- 

 lowed the usual determinations, but I am not satisfied with them. The 

 "supraorbital," though occupying the position usual for this bone 

 above the orbit, has relations anteriorly that are altogether unusual ; 

 the nasal and the lachrymal do not appear to exist as independent 

 elements. It would seem more likely that this supposed "supra- 

 orbital" is really the lachrymal, if the postfrontal and prefrontal are 

 fused into one element. Again, such a combination of the postfrontal 

 and prefrontal and their peculiar articulations is remarkable. The 

 very narrow frontal, while showing a distinct suture in the middle, 

 presents no evidence of any connection with the parietal — it seems 

 more to be a very narrow rostrum projecting in front of the parietal 

 and separating the bones, which otherwise would answer very well 

 for frontals. In this latter case, the so-called "supraorbital" would 

 really be the prefrontal, and the postorbital the postfrontal or post- 

 fronto-orbital. This may seem a violent supposition, but it does not 

 seem at all improbable to me. Nor is the union of the parietal with 

 the premaxilla any more extraordinary than is the union of the supra- 

 occipital with the frontal in many Cetacea. 



Sclerotic plates are present in the left orbit of this specimen in a 



