24 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.63, 



In regard to the slight differences which are observable in the 

 tympanic and periotic bones of this fossil porpoise and Platanista^ 

 one is encouraged to conclude that there must be a closer relationship 

 existing between these dolphins than v/ith the Delphinidae. 



Measurements of the tympanic. 



mm. 



Greatest length of left tympanic bulla 52. 5 



Gi'eatest width of right tympanic bulla 27.6 



Greatest depth of right tympanic bulla on internal side 20 



Greatest depth of right tympanic bulla on external side (ventral face to 

 tip of processus sigmoidcus 36.5 



MANDIBLES. 



One distinguishing feature of the combined lower jaws (pi. 3) of 

 this fossil porpoise, as compared with jaws of Platanista, Inia, and 

 Lipofes, is the great length of the symphysis. The free portion of 

 either mandible is less than one-third of its total length. The com- 

 bined lower jaws taper toward the tip, the width at the proximal end 

 of the symphysis being equal to more than four times the width at 

 extremity. The depth of either mandible at proximal end of the 

 symphysis is nearly three times that at the extremity. There is a 

 conspicuous median longitudinal groove between the tooth rows on 

 the posterior one-half of the symphysis. The distance from the sym- 

 physis to last tooth is much less than distance between same tooth of 

 opposite rows. 



Back of the symphysis the ramus consists mainly of a thin shell 

 of bone. The external wall of the ramus is continued backward to 

 form the coronoid process, the condyle, and the angle. The internal 

 wall of the right ramus terminates 183 mm. behind the symphysis; 

 at this point the inferior dental canal enters the mandible. Between 

 the symphysis and the terminus of the internal wall the ramus is 

 hollow ; the depth of the cavity at the proximal end as estimated is 

 equal to approximatelj^ four times that at the symphysis. 



The superior margin of the mandible gradually rises from the 

 symphysis to the coronoid, and is accompanied by a downward cur- 

 vature of .the inferior margin. In consequence the coronoid as origi- 

 nally preserved was deep and somewhat convex on the external face. 

 As a result of crushing, the posterior one-third of both mandibles 

 show a longitudinal fracture at the level of the condyle. The lower 

 portions of each of these mandibles as shown on plate 3 are thus 

 pushed inward and lie in a horizontal position. For this reason 

 some allowance must be made in estimating the depth of the mandi- 

 ble at the coronoid. It appears that the depth through the coronoid 

 (135 mm. estimate) is equal to less than one-half of the free portion 

 of the left mandible. The coronoid is broadly rounded, while the 



