16 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.63. 



contact with the internal pterygoid, a flattened wedge-like bone which 

 is in contact externally with the external peterygoid. The opening 

 for the infraorbital canal appears in the maxillary bone in front of 

 the nasal passages but posterior to the maxillary notch. 



On the ventral surface of the skull the vomer surrounds the pre- 

 phenoid and entirely conceals it from view. It extends backward 

 across the basisphenoid, but, since the basicranial portion of this 

 skull is missing, the posterior limit can not be determined. The 

 vomer is deepest at the nasal passages and in consequence this por- 

 tion of the skull is characterized by a prominent keel. This keel 

 increases in height posteriorly and attains its greatest depth imme- 

 diately in front of the nasal passages; it flattens out anteriorly at 

 the level of the last tooth. The concave areas on the maxilla on either 

 side of this keel extend forward to the tooth rows, while in Lipofes 

 they terminate 155 mm, posterior to the tooth roAvs. 



The peculiar features and modifications of the bones surrounding 

 the above-described keel may be compared with PJatanista and 

 Lipotes. Unfortunately, some uncertainty exists as to whether or 

 not the type skull of the Chinese river dolphin ^ has been damaged. 

 The irregular edges of the descending plates or fortuitous projections 

 of the maxillae indicate that part of these bones are missing. If they 

 were complete, they would inclose pyramidal cavities similar to those 

 possessed by this fossil skull. The relations between the vomer, the 

 internal pterygoids, and the palatines are essentially the same in 

 all three genera. It should be noted that the maxillae of PJatanista 

 and Lipotes meet mesially in a linear suture at level of last tooth 

 and thus conceal the keel of the vomer in front of the nasal passages. 

 In this fossil porpoise the keel of the vomer appears between the 

 maxillae. 



On the left side of this fossil skull (pi. 2) the external pterygoid 

 extends forward 112 mm. in advance of the posterior wall of the 

 nasal passage. This bone also extends backward beyond the nasal 

 passage; it is bounded by the maxilla anteriorly, by the squamosal 

 posteriorly, and by the frontal superiorly. In these respects the 

 approach is directly toward the relations existing between these 

 bones in the skull of PJatanista (pi. 6). One of the distinguishing 

 features of skulls of PJatanista and of this fossil porpoise, as com- 

 pared with skulls of Lipotes and Inia, is the expansion of the ex- 

 ternal pterygoid. In consequence the alisphenoid is not exposed on 

 the side of the skull in the temporal fossa. 



'Miller, G. S., jr., A new river dolphin from Cliina. Smithsonian Misc. Coil., vol. 68. 

 No. 9, Publ. 2486. pi. 6, Washington, 1918. 



