12 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.63. 



overhanging premaxillae, this interpretation is based solely on the 

 short interval of mesorostral gutter exposed in front of the pre- 

 sphenoid. In this region the vomer forms the floor of the gutter and 

 takes part in the formation of the lateral walls. If any reliance can 

 be placed upon the anterior limit of the exposed vomer as seen from 

 the ventral view of the rostrum and upon analogous relations of the 

 same element in the skull of Lipotes^ then the vomer disappears 

 in the floor of the mesorostral gutter about 240 mm. in front of the 

 maxillary notches. It appears that the vomer contributes the 

 greater part of either wall of the mesorostral gutter in front of the 

 presjDhenoid, but the corresponding surfaces of vomer and premax- 

 illae are so smoothly mortised into one another that the actual line 

 of contact can not be determined with any degree of accuracy. It 

 is evident, however, that the contact between the vomer and either 

 premaxilla has its posterior limit near the anterior end of the pre- 

 sphenoid. The dorsal margins of the mesorostral gutter are formed 

 by thin plates of the premaxillae which project inward from the 

 raised convex outer portions and whose edges are deflected obliquely 

 upward. 



The premaxillae do not closely approximate each other above the 

 presphenoid to form a slit-like anterior border for the nasal aper- 

 tures as in Li'potes. In consequence most of the anterior end of the 

 presphenoid is visible. This porous bone forms a plug across the 

 proximal end of the mesorostral gutter and rises to the level of tli3 

 premaxillae above. In this last-mentioned feature, however, the 

 skull of this fossil porpoise agrees more closely with Lipotcs than 

 with Inia. 



All of the brain case posterior to the nasal passages, with the ex- 

 ception of a small portion which comprises the vertex, was missing 

 when the skull was excavated. Unfortunately the ascending por- 

 tion of the mesethmoid also has been largely destro3^ed. The small 

 fragments of this bone which still adhere to the dorsal surface of 

 the presphenoid show that the mesethmoid forms the partition sep- 

 arating the nasal passages superiorly. Both nasal passages of this 

 fossil skull are well preserved and one is thus permitted to de- 

 scribe these structures in considerable detail. After a most thorough 

 comparison with Platanista, Lipotes, and /ma, it was found that 

 the relations of the various elements which enter into the forma- 

 tion of these passages are essentially the same in all. The m?seth- 

 moid sheathes or forms a thin veneer of bone around the dorsal 

 face and the upper halves of the lateral faces of the presphenoid, 

 conceals the frontal fontanelle, and extends downward in either 

 nasal passage to meet the ascending process of the vomer. On the 

 base of the skull the vomer extends backward across the basisphenoid. 



