10 PROCEEDINGS OF THE NATIONAL MUSEUM, vol.63. 



The posterior end, or apophysis, is elongate and expanded, and is 

 produced externally as in most whalebone whales. The apophysis 

 fills the cavity between the exoccipital and the posterior margin of 

 the groove for the external auditory meatus. The posterior pedicle 

 of the tympanic is fused to the apophysis. The pedicle on the an- 

 terior end of the tympanic is fused with the pro-otic and the point of 

 contact (pi, 5, fig. 4) is situated in advance of the labyrinthic portion 

 of the periotic. The anterior process, or pro-otic, is very large in 

 proportion to the labyrinthic portion. The antero-external corner 

 of the anterior process is rounded and the antero-internal is produced 

 inward, forming a wedge-shaped plate or lamina. On the ventral 

 face of the pro-otic (pi. 5, fig. 3) and opposite the fenestra ovalis is 

 a circular depression in which the head of the malleus is lodged. 



The internal wall of the opisthotic descends obliquely inward while 

 the external wall of the labyrinthic portion of the periotic is more 

 nearly vertical. Between these two closely approximated faces and 

 at the top of the sinus which they form lies the semi-inclosed groove 

 for the facial nerve. The groove which marks the course of the nerve 

 is 10 mm. above the inferior face of the opisthotic and 8.5 mm. above 

 the corresponding surface of the labyrinthic. The groove for the 

 facial nerve begins at the posterior margin of the periotic and, curv- 

 ing inward, leads to the epitympanic orifice of the facial canal. The 

 fenestra ovalis is situated on the ventral face of the labyrinthic por- 

 tion of the periotic and on a level with the internal margin of the 

 above-mentioned groove. The foot plate of the stapes is firmly im- 

 bedded in the fenestra ovalis; the remainder is missing. The incus 

 was lost, possibly during preparation, though the malleus (pi. 5, fig. 

 4) was in place when the skull was placed in the writer's hands for 

 description. 



The postero-external face of the labyrinthic portion projects back- 

 ward as a thin sheet or lamina of bone ; internal to this process and 

 on the posterior face there is a circular opening, the fenestra ro- 

 tundum. 



The most noticeable feature of this periotic is the circular con- 

 cavity on the dorsal face, which, coupled with the location of the 

 internal foramina, impart an unusual appearance to this bone. As 

 seen from the dorsal view (pi. 5, fig. 1), the labyrinthic region is 

 roughly triangular. Below the apex of this structure is a large 

 elliptical opening, which is divided mesially by a thin, bony parti- 

 tion. The posterior fossa represents the internal acoustic meatus, 

 at the bottom of which are three small circular depressions. The 

 more internal one of these leads to a small opening, presumably the 

 foramen centrale. The spiral tract is represented by the two remain- 

 ing depressions. The fossa is approximately 7 mm. deep. The pas- 

 sage anterior to the internal acoustic meatus represent the internal 



