114 PROCEEDINGS OF THE NATIONAL MUSEUM vol. los 



half of the symphysis curves distinctly upward in contrast to the 

 general downward slope of the corresponding portion of the mandibles 

 of Inia. The anterior end of the symphysis is obliquely truncated 

 in a dorsoventral direction. The opposite free posterior portions of 

 the mandibles come together at a 28-degree angle at the symphysis. 



About 61.5 percent of the total length of the left mandible is 

 occupied by the tooth row in contrast to 66 percent for Inia. The 

 number of alveolae, however, is approximately the same in these 

 fossil and Recent porpoises. The fossil mandibles have 26 alveolae in 

 each ramus, the five alveolae at the anterior end of each ramus being 

 somewhat larger than the others. The tendency toward the antero- 

 posterior elongation of the alveolae in the symphyseal region, so 

 noticeable in Saurocetes argentinus (Rovereto, 1915, pi. 2, fig. 1; 

 Burmeister, 1871) and to a lesser degree in Goniodelphis hudsoni 

 (Kellogg, 1944, pi. 1, fig. 1; Allen, 1941), is hardly discernible. The 

 alveolae in the left mandible measure from 8 to 15 nun. anteropos- 

 teriorly and from 8 to 12 mm. transversely. The 17 alveolae on the 

 hinder portion of each mandibular tooth row are separated by rather 

 thin septa, while those situated near the middle of the symphysis are 

 separated by intervals of 5 to 7 mm. The presence of 14 teeth in 

 each ramus behind the level of the hinder end of the symphysis 

 readily distinguishes these mandibles from those of Inia geqffrensis 

 (USNM 49582). As a result of the lengthening of the symphysis 

 on the mandibles of Inia, there are only 5 teeth in the left ramus and 

 6 teeth in the right ramus posterior to the hinder end of the symphysis, 

 although there are 27 teeth in each mandible. 



Opposite alveolae at hinder end of symphysis are separated by an 

 interval of 26 mm.; the corresponding interval between the 25th 

 alveolae (counting forward from hindmost) is 11 mm. The symphysis 

 is semicircular in cross section. 



On the outer surface of the left mandible (pi. 4, fig. 2), and about 

 16 mm. below the rim of the ninth alveola (counting forward from 

 the hindmost) is a foramen of moderate size. The next external 

 nutrient foramen is smaller and is located about 14 mm. below the 

 septum separating the 15th and 16th alveolae. Two small foramina, 

 one situated above the other, are located 10 mm. below the rim of 

 the 19th alveola. The anteriormost foramen is located 17 mm. below 

 the rim of the 21st alveola. From each of these alveolae shallow 

 grooves of varying lengths extend forward. The anterior 175 mm. of 

 the outer surface of the left ramus is quite rough and nodular, in- 

 dicating that at least this portion of the ramus was covered with a 

 tight-fitting skin. 



The conformation of the posterior portions of the mandibles of 

 this fossil porpoise insofar as preserved are somewhat similar to 



