28 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 69 



relations between this apophysis and the sigmoid process are es- 

 sentially the same as in Prodelphinm malayanus. 



The posterior process is borne on a shorter neck than in Prodel- 

 phinus malayanus and the articular facet is considerably larger. 

 The wide involuted portion (pi. 1, fig. 4) of the tympanic is "de- 

 pressed below the level of the arching thin outer lip and gradually 

 narrows as it approaches the anterior outlet of the Eustachian canal. 

 The surface of the involucrum (pi. 1, fig. 5) is relatively smooth, 

 convex from side to side, and depressed mesially. The involucrum 

 of the tympanic bulla of ProdelpUnus malayanus "is depressed in 

 front of the posterior process. 



The thin outer lip of the left tympanic bulla (pi. 1, fig. 5) is 

 sufficiently well preserved to show the size and direction of ''the an- 

 terior outlet of the Eustachian canal. The anterior end of the bulla 

 is slightly produced, forming a narrow lip which projects forward. 

 The superior border of the thin outer lip of the bulla turns and curves 

 inward, forming a narrow shelf. The accessory ossicle or uncinate 

 process of the tympanic is no larger than that of Prodelphinus malay- 

 anus, measuring 7.8 mm. in length, 6.5 mm. in width, and 3.8 mm. in 

 depth. The dorsal surface of this ossicle is traversed by a shallow 

 mesial groove which curves from end to end. The internal portion 

 of the accessory ossicle is larger, thicker, and more nearly ovoidal in 

 outline than that of Prodelphinus malayanus. The dorsal margin 

 of the thin outer lip of the tympanic bulla is fused with the ventral 

 surface of the accessory ossicle along the external margin of this 

 ovoidal internal structure. 



The ventral surface of the tympanic bulla (pi. 1, fig. 2) is slightly 

 depressed mesially. There is a short groove on the anterior end 

 which becomes less distinct as it approaches the mesial depression, 

 but the posterior end is characterized by a large groove which is fully 

 4 mm. in depth at the posterior margin. When viewed from the ven- 

 tral face, the inner margin is seen to be biconvex and the outer mar- 

 gin convexo-concave. 



Measut-ements of left tijmpanic (in millimeters) 



Greatest length of bulla 29. 7-4- 



Greatest depth of bulla on internal side (ventral face to dorsal face of 



involucrum g g 



Greatest depth of bulla on external side (ventral face to tip of posterior 



apophysis) Ig 3^ 



Greatest width of involucrum 94 



MALLEUS 



The head of the left malleus (fig. 12) is of the same form as in 

 Prodelphinus malayanus (Cat. No. 36051, United States National 

 Museum) and the upper facet is larger than the lower. These 



