SKULLS of-' THE rnVGhODOKTiVAi. 



649 



as to their detennination and relationship to one another. For 

 the purposes of the present paper these may be disregarded and 

 the series represented by Prozeuglodon atrox, Zeuglodon inter- 

 medius, and Zeuglodon osiris alone considered. The characters of 

 the skulls of these species completely support ]>. Dart's conclu- 

 sion, founded on the endocranial casts, that these three species 

 represent terms in a phyletic series. 



The skull of Prozeuglodon has been described in detail in the 

 catalogue (loc. cit.), so that here it Avill only be necessary to refer 

 to a few characters easily comparable with those of the other 

 forms. The occipital surface (text-fig. 22) above the foramen 

 magnum is wider thnn high (if the height* be taken as 100, the 

 width is 110) and it is only gently concave from side to side. The 

 foramen magnum and the occipital condyles are both large. The 

 convexity of the portion of the cranial wall formed by the parietal 



Text-figure 22. 



Lambdoid crest 



Scjuamcus portion 

 of tervpora/. 



Masto/d 



process of I — 



petrosa/. Psroccipital process 



Occipital condy/e 



magnufn. 



Occipital surface of skull of Frozeiicflodoti atrox Andrews. M. 9266. 

 About 5 iiat. size. 



and squamosal is greater than in the later forms, while the sagittal 

 crest is much less developed. The posterior border of the narial 

 opening is over the space between the first and second premolars 

 and the snout is less prolonged anteriorly than in the Inter 

 forms. Some of the above peculiarities may be dependent to 

 some extent on the circumstance that the skull described is not 

 quite adult. 



The type-skull M. 10173 of Zeuglodon intermedius (text- fig. 23) 

 is not complete, the zygomatic arch and the terminal portion of 

 the snout being missing ; the teeth also are much broken. In it 

 the width of the occipital surface (text-fig. 23) is much less 

 in i)roportion to its height* (height 100, width 66), and it is 

 much more concave from side to side than in Prozeuglodon. 



* In each case tlic height is measui'ed from the upper border of the foramen 

 magnum to the point of union of the sagittal and lambdoidal crests. 



