12 P.ROCEEDINGS OF THE NATIONAL MUSEUM vol.69 



but in front of the antorbital notches they commence to slope more 

 and more from the inner to the outer margin and on the distal one- 

 third of the rostrum are almost vertical; they decrease in breadth 

 but increase in height toward the extremity of the rostum. The 

 inner margins of the premaxillae are almost in contact with one 

 .another in front of the respiratory passages for a distance of about 

 15 mm. and, after spreading apart, gradually converge again up to 

 a point 90 mm. in front of tlie antorbital notches, and then diverge 

 even more widely than on the basal half of the rostrum. The pre- 

 maxillae commence to expand horizontally near the middle of the 

 rostrum and attain their maximum breadth at the level of the an- 

 terior border of the respiratory passages. Opposite to these pas- 

 sages each premaxilla is raised above the corresponding maxilla 

 and the outer border slopes to the maxillary suture. The posterior 

 •extremity of each premaxilla is bluntly acuminate and meets the 

 antero-external face of the five-sided nasal edge to edge. The pre- 

 •maxillary foramina are rather large and are situated posterior to 

 the antorbital notches and anterior to the maxillary foramina. 

 Two narrow grooves lead from each of these premaxillary foramina, 

 ,one of which extends obliquely forward to the internal margin ; the 

 other, a much deeper groove, curves backward and outward and is 

 ■.continued posteriorly for some distance along the external margin of 

 the premaxilla. Anterior to the premaxillary foramen the internal 

 jDortion of the premaxilla is set off from the external border by the 

 first-mentioned groove; this triangular strip narrows rapidly and 

 finally disappears in consequence of the arching of this bone. 



With the exception of a short interval in front of the respiratory 

 passages where the inner margins of the premaxillae are almost in 

 contact, the mesorostral gutter is open for its whole length. On the 

 distal end of the rostrum the premaxillae meet mesially and ventrally 

 jn a linear suture and form the floor and sides of the mesorostral 

 gutter; on the proximal half the vomer and premaxillae contribute 

 ;to its formation. The vomer increases in Avidth from its anterior 

 end to the respiratory passages and near the proximal end of the 

 mesorostral gutter rises to the level of the pluglike presphenoid. 



As in the living southern porpoise, Sotalia tucuxi (Cat. No. 21499, 

 United States National Museum), the mesethmoid is limited to the 

 mesial longitudinal strip of bone which constitutes the most dorsal 

 portion of the wall between the respiratory passages and the sutures 

 which mark its contact with the laterally placed ectethmoids have 

 disappeared. These sutures likewise disappear with age in living 

 porpoises. A continuous sheet of bone extends upward, overspread- 

 ing the lower borders of the anterior faces of the nasals and the 

 internal borders of the more or less vertical plates of the f rontals, 



