MIOCENE PORPOISES — KELLOGG 133 



MANDIBLES 



The anterior extremities of both mandibular rami (pi. 12, fig, 1) 

 are broken off; the condyle and a portion of the ramus below and 

 behind the coronoid process is missing on the right mandible; the 

 left mandible is essentially complete except for the lower border in 

 front of the condyle. In the present condition, the symphyseal 

 portion (229 mm.) of the left mandible is shorter than the hinder 

 free portion (307 mm.). The mandibles are firmly ankylosed through- 

 out the length of the symphysis. A thin dorsal groove along the 

 midline on the anterior half of the symphysis marks the ankylosis 

 of the opposite mandibles. On the posterior half of the symphysis 

 this groove is more or less obliterated. On the ventral face the line 

 of fusion is indicated by a continuous narrow groove that extends 

 medially the full length of the symphysis. 



The symphyseal portion of the combined lower jaws tapers toward 

 the anterior extremity, the transverse diameter diminishing from 

 52.5 mm. at the level of the posterior end to 25.4 mm. at the level of 

 the 29th alveola, counting forward from the hindmost alveola in the 

 ramus, although the dorsoventral reduction is less marked. From 

 a lateral view (pi. 13, fig. 2) the ventral profile of the distal 130 mm. of 

 the symphysis slopes slightly upward toward the missing anterior 

 extremity. Between the tooth rows the dorsal surface of the sym- 

 physis is relativelj^ smooth. No pits for the reception of the apices 

 of the teeth in the upper jaws are present. The external face of the 

 symphyseal portion of the right mandible is slightly convex, but slopes 

 obliquely from the alveolar margin to the midline of the ventral 

 surface. A cross section taken about half way of the length of the 

 ankylosed symphyseal portions of the mandibles resembles a U, 

 but is more nearly V shaped near the extremity. 



The distance (117 mm.) from the posterior end of the sjrmphysis to 

 the posterior margin of the hindmost alveola on the right mandible is 

 greater than the interval (77 mm.) between the opposite tooth rows 

 at the level of this tooth. The angle formed by the diverging op- 

 posite rami behind the symphysis is approximately 30 degrees. 



More than 33 alveolae were present on the left mandible when 

 complete. If the extremity of the symphysis had been preserved 

 the number of alveolae would be increased, probably at least to 36. 

 This extinct porpoise had originally more than 21 teeth on each side 

 of the ankylosed symphysis, as contrasted to 12 in the case of Lopho- 

 cetus pappus, new species. The teeth on the anterior half of the 

 symphysis were implanted opposite one another, the alveolae being 

 separated by equivalent intervals from preceding and succeeding 



338378—55- 



