PEOZEUGLODON. 249 



Internal to the articular surface, the squamosal unites in suture with the alisphenoid, 

 and appears to send down a flange of bone which helped to support the great swollen 

 tympanic. This element is badly preserved in the available specimens, but it can be 

 seen that it formed a great egg-like expansion with very thick walls and seems to 

 be obscurely divided into a small postero-internal lobe and a very much larger outer 

 one ; it is wedged in between the squamosal on the outer side, the basioccipital and 

 C?) the basisphenoid on the inner, and the alisphenoid in front. The periotic (per.), 

 as already described, appears on the posterior surface of the skull between the 

 exoccipital and the squamosal ; on the inner face of the skull it extends some distance 

 upwards. The external auditory meatus opened immediately behind the postglenoid 

 process of the squamosal. 



The alisphenoid [al.) must have united at its lower end with the basisphenoid, but the 

 suture is obliterated. Immediately in front of the tympanic bulla it is perforated by 

 a large foramen, apparently the foramen ovale. Above this it joins and is overlapped 

 by the inner end of the portion of the squamosal bearing the articular surface. Above 

 this again it runs obliquely upwards and forwards on the side of the cranium as a 

 wing of bone, which unites behind with the parietal and above with the frontal for 

 a short distance. The upper part of its anterior border forms the outer lip of the 

 posterior, end of the deep groove for the optic nerve [opt.), which runs forwards and 

 upwards, the upper edge of the groove being apparently formed by the lower border 

 of the frontal and its floor by the orbitosphenoid (as.). The lower part of the anterior 

 edge of the alisphenoid seems to unite with the posterior edge of the orbital plate 

 of the palatine, which takes a large share in the formation of the side wall of the skull, 

 and along its upper edge overlaps the orbitosphenoid. To the inner side of and 

 slightly below the level of the orbit there is a large orbito-nasal (spheno-palatine) 

 opening (PI. XXI. fig. 1 D, o.n.f.), which appears to perforate the orbital plate of the 

 palatine and communicates with the nasal passage. 



The foramina of the skull are not well seen. There is a distinct condylar 

 foramen in the exoccipital. The foramen lacerum posterius probably occupied the 

 interval between the basioccipital and exoccipital and the tympanic. The lower 

 end of the alisphenoid is perforated by a foramen which may be equivalent to the 

 foramen ovale. The foramen lacerum anterius and the optic foramen must have 

 opened behind the edge of the alisphenoid at the bottom of the deep groove which 

 runs forwards to the orbit as above mentioned. The relations of the lower part of 

 the palatines and- the pterygoids cannot be made out. 



From the above description it will be seen that the skull in Prozeuglodon diff'ers in 

 no important particulars from the skulls of Frotocetus atavus and Zenglodon osiris, 

 so well described by Drs. E. Fraas and Stromer respectively. It is, in fact, in some 

 ways intermediate between the two, as, for instance, in the position of the nares. 

 Thus, if the total length of the skull be taken as 100, then in Frotocetus (assuming 



2k 



