92 



BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



Fig. C. — Left tynipano-perioticof the exist- 

 ing Delphinapterus leucas Pallas. luuer or 

 cerebral face, inverted, x i- 



for the nervus facialis is confluent with tlie porus acusticus on that side 

 of the periotic which is turned away from the tympanic, but it sometimes 

 happens, as in Mesoplodon, that these openings are separated. The 

 nervus facialis emerges on that side of the periotic which is turned toward 



the tympanic through a small 

 round aperture (apertura tym- 

 panica canalis Fallopiae) which 

 is continued backward in the 

 form of a semi-enclosed canal 

 (20). Two other openings are 

 visible on the cerebral face of 

 the periotic, placed one above 

 the other in close proximity to 

 the porus acusticus. The smaller 

 of these communicates directly 

 with the cochlear labyrinth, and 

 is only separated from the ex- 

 ternal opening of the latter 

 (^fenestra cochleae) by a slight 

 bridge. The name given to this 

 smaller opening is apertura externa aquaeductus cochleae (14). The 

 larger one above it is called from its communication with the scala 

 vestibuli the apertura externa aquaeductus vestihidi (13). All of these 

 openings vary more or 

 less in form, size, and 

 position amongst differ- 

 ent genera, and hence 

 require attentive exam- 

 ination. 



Inferior face (Fig. D). 

 — Rotation throutrh an- 

 other quadrant of arc 

 brings into view the 

 lower surface of the 

 bulla ossea. It is trav- 

 ersed longitudinally 

 along its middle portion 



by a broad sulcus that gradually deepens posteriorly, thus forming an 

 inner (16) and an outer (1) lip. The surface of the latter is smooth, 

 that of the former very rough, the rugosity extending about half-way 



/9 



--// 



Fig. D. — Left tympano-periotic of the existing Del- 

 phinapterus leucas Pallas. Inferior face, x x- 



