JVKT. 14. MARYLAND FOSSIL PORPOISE— KELLOGG. 23 



Although imperfect, the left tympanic bone of this fossil porpoise 

 is sufficiently entire to show the size and direction of the anterior 

 outlet or the tympanic aperture of the eustachian canal. The an- 

 terior end of the tympanic is drawn out into a narrow laterally com- 

 pressed process which is directed forward and downward. This 

 process is missing on the right tympanic (pi. 7, fig. 4), but the thin 

 outer lip is practically entire. In Platanista (pi. 7, fig. 3) the 

 inferior margin of the outer lip of the bulla turns abruptly and 

 curves inward, forming a shelf. This modification, apparently, was 

 not present on the tympanic of this fossil porpoise. 



The tympanic cavity, which is bounded by the overarching outer 

 lip and by the involucrum, is essentially similar to that of Platanista. 

 The anterior process of the tympanic, which unites with the peri otic, 

 is broken off at the level of the outer lip. When viewed from the 

 external side, the posterior margin of the tympanic is seen to be more 

 rounded than in Platanista^ the processus sigmoideus is longer, and 

 the tympanic as a whole is relatively deeper. The processus sig- 

 moideus of the right tympanic (pi. 8, fig. 2) is entire, the terminal 

 half being twisted at right angles to the basal. The groove on the 

 external face of the tympanic anterior to the processus sigmoideus 

 is relatively broader than in Platanista (pi. 8, fig. 1). The so-called 

 posterior conical apophysis is shorter than in Platanista, but other- 

 wise the relations between this apophysis and the processus sig- 

 -moideus are essentially the same in both genera. The apophysis 

 projects above the level of the superior face of the involucrum. 



The posterior process (pi. 7, fig. 2) is broken off at the level of the 

 involucrum. The broken edges show that it projected from the pos- 

 terior end of the tympanic and that the out»r lip and the involucrum 

 contributed to its formation as in Platanista. The thick convex 

 involuted portion of the tympanic is slightly and unequally de- 

 pressed below the level of the overarching outer lip and subsides 

 rather abruptly just posterior to level of the anterior process of the 

 outer lip, while the anterior portion becomes decidedly concave in- 

 ternally. The surface of the thickened or posterior portion of the 

 involucrum is constricted or depressed medially on its internal and 

 dorsal faces. 



The ventral surface of the tympanic exhibits a deep groove which 

 is most pronounced near the posterior margin. In Platanista (pi. 

 8, fig. 3), however, this groove is not open, but is filled in with 

 spongy bone. When viewed from the ventral side the outer margin 

 of the bulla (pi. 8, fig. 4) is seen to be biconvex and much broader 

 anteriorly than in Platanista. The anterior and posterior margins 

 of the bulla do not slope as strongly from the external to internal 

 faces as they do in the living genus. 



