18 PROCEEDINGS OF THE NATIONAL MUSEUM. voi.. o:?, 



able in the size and relations of the various bones which comprise the 

 outer wall of the cranium. In these details, however, the approach 

 is directly toward the conditions existing in the skull of Platanista. 

 In the latter, however, the position of the lachrymal with reference 

 to the inferior opening of the infraorbital canal is somewhat differ- 

 ent. The squamosal is relatively large, but the internal portion 

 which forms part of the outer wall of the cranial cavity is largely 

 missing. The lateral projection of the squamosal forms the forward 

 projecting zygomatic process and the downward projecting postgle- 

 noid process. The left zygomatic process is complete; its greatest 

 width is 61.5 mm. and its greatest length is approximately 100 mm. 

 As seen from the ventral view, the zygoma is robust and short in 

 comparison with Inia. On the other hand, the zygoma bears a close 

 resemblance to that of Platanista and this similarity is accentuated 

 by the contact between it and the supraorbital process of the frontal. 



The glenoid surface extends forward upon the zygomatic process ; 

 it is rather wide, concave antero-posteriorly, and is limited externally 

 and anteriorly by a distinct outer margin. The external auditory 

 meatus, which commences at the postero-internal margin of the 

 squamosal, apparently, does not wind around the postglenoid process 

 of that bone for no groove can be found. The postglenoid process 

 is directed backward and downward and does not curve forward as 

 in Lipotes. The posterior portion of the glenoid articular surface 

 of the Chinese river dolphin skull is deeply concave ; in consequence 

 the articulation with the lower jaw is restricted to a narrow area. 

 Such is not the case with this fossil skull, for this portion of the 

 glenoid articular surface is almost flat. On the left squamosal at 

 the inner side of the glenoid area there is a shallow groove which 

 commences behind the postglenoid process, but which does not ex- 

 tend forward to the anterior margin of the glenoid process of the 

 squamosal as in Lipotes. 



The origin of or possible use for a peculiar shelf formed between 

 the upper margin of the squamosal and the frontal is difficult to 

 explain. A similar groove is present in the skull of a young 

 Platanista (Cat. No. 172409, U. S. Nat. Mus.), but this structure 

 does not occur in Inia or Lipotes. In this fossil skull the shelf 

 and groove (pi. 4) formed by it extends forward to the anterior 

 margin of the supraorbital process of the frontal; it is thus bounded 

 inferiorly by the squamosal and the external pterygoid. The groove 

 is much shorter in the Platanista skull and terminates anteriorly 

 at the level of apex of glenoid portion of squamosal; the foramen 

 ovale opens into this groove. In the left temporal fossa of this 



