106 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 105 



pinched in externally below this crest. The posterior end of each 

 premaxillary is in contact with the upturned inner edge of the corre- 

 sponding ascending plate of the maxillary and internally for a distance 

 of about 15 mm. abuts against the corresponding nasal bone. The 

 premaxillary foramina are elongated and are situated slightly behind 

 the level of the antorbital notches. Two narrow grooves lead from 

 each of these premaxillary foramina, the outer one extending obliquely 

 backward and outward, and the inner shorter one curving backward 

 and inward to near the internal edge of this bone. Commencing at 

 about the level of this premaxillary foramen and extending backward 

 is a shallow groove which follows more or less the curvature of the 

 outer edge of the premaxillary about 14 mm. inside the latter. 



With the exception of a short interval in front of the nasal passages 

 where the inner margins of the premaxillaries are almost if not in 

 actual contact, the mesorostral gutter seems to have been almost 

 completely roofed over by the close approximation of the opposite 

 inner margins of these elongated bones. On the distal 230 mm., the 

 premaxillaries meet on the midline of the palatal surface of the 

 rostrum, and constitute the floor and sides of the mesorostral gutter; 

 behind this point the vomer and the premaxillaries contribute to its 

 formation. The vomer increases in width from its anterior end to in 

 front of the nasal passages where it is applied to the lateral surfaces 

 of the presphenoid. The presphenoid appears to be rather porous and 

 forms a plug at the hinder end of the mesorostral gutter. 



As in the Recent South American river porpoise Inia geoffrensis 

 (USNM 239667, cf), the mesethmoid is limited to the median longi- 

 tudinal strip of bone which constitutes the dorsal portion of the wall 

 between the nasal passages. The mesethmoid also sheathes the dorsal 

 and upper halves of the lateral faces of the laterally compressed 

 presphenoid and on the inner wall of each nasal passage meets edge to 

 edge the corresponding wall of the troughlike vomer in which the 

 presphenoid rests. All traces of sutures that mark the contact of the 

 mesethmoid with the laterally placed ectethmoids have disappeared 

 with ankylosis. These fused ethmoid bones form a continuous sheet 

 of bone, now destroyed dorsally, that overspread the ventral halves of 

 the anterior faces of the nasals and completely closed the area through 

 which the olfactory nerves formerly passed. 



A U-shaped anterior border of the combined nasal passages is formed 

 by the close approximation of the internal edges of the opposite 

 premaxillaries. At the level of the vertex, the posterior wall of each 

 nasal passage is approximately on a line with the ends of the post- 

 orbital projections of the supraorbital processes. The posterior wall 

 of each nasal passage does not descend obliquely as in Inia, but about 



