116 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 105 



anterior process for the reception of the uncinate process of the outer 

 lip of the bulla has a somewhat different conformation. 



The articular facet on the ventral surface of the posterior process 

 is incomplete on all sides. This facet is shallowly concave on the 

 basal half and the articular surface slopes from the outer to the an- 

 terior margin. Two low ridges extending from the outer to the inner 

 margin are present on the hinder half of this facet. The anterior 

 face of the posterior process is excavated; the external face has been 

 sheared off; and the posterior face is irregular in contour. 



As regards the ventral aspect of the pars cochlearis, the resemblance 

 to Lophocetus calvertensis is not very close, since the latter is notice- 

 ably larger and the curvature of this surface is more regularly convex. 

 The facet for the accessory ossicle or uncinate process of the tympanic 

 bulla on the ventral surface of the anterior process is deeply concave 

 and is limited internally by a deep narrow groove. The fenestra 

 rotunda is roughly subtriangular in outline and larger than the fenestra 

 ovalis. The foot plate of the stapes completely fills the ovoidal 

 fenestra ovalis and is held in position by narrow internal ledges on 

 the anterior and posterior walls. Within the fenestra ovalis are the 

 orifices of the small semicircular canals that open into a circular de- 

 pression in the vestibule opposite to the epitympanic orifice of the 

 aquaeductus Fallopii; a larger orifice situated at the posterointernal 

 angle of the vestibule is the terminus of the aqueduct leading from 

 the foramen rotundum and the scala vestibuli. 



The epitympanic orifice of the aquaeductus Fallopii is small and the 

 narrow groove for the facial nerve, which leads from it, is definitely 

 limited by the thin rim of the fenestra ovalis and the projecting ledge 

 formed by the internal margin of the fossa for the head of the malleus, 

 but posterior to these structures its course along the internal and pos- 

 terior faces of the posterior process is less sharply defined. The 

 elongate fossa for the stapedial muscle, which is located in a depression 

 behind the fenestra ovalis and internal to the groove for the facial 

 nerve, is rather deep and somewhat rugose. Along the internal margin 

 of the stapedial fossa a low thin-edged crest is developed on the ventro- 

 external angle of the pars coclilearis, which extends backward to the 

 base of the posterior process. The depth of the posterior face of this 

 periotic (11.5 mm.), as measured from the stapedial fossa to the dorsal 

 face above the cerebral orifice of the aquaeductus vestibuli, is some- 

 what less than the corresponding measurement of the periotic of 

 Lophocetus calvertensis (12.1 mm.). 



Between the rounded tuberosity or swelling, on which the fossa for 

 the head of the malleus (pi. 5, fig. 1) is located, and the anterior margin 

 of the articular facet on the posterior process the ventral surface of the 



