SCHULTE, SKI WHALE. 495 



extent to which the displacement of the otic capsule from its primitive position in the side wall 

 of the cranium, toward a basal position has proceeded in Balcenoptera. These surfaces are 

 demarcated by the convex edge of the first turn of the cochlea, except dorso-laterally, where 

 the border is interrupted by the junction with the pars canalicularis. In this region the dorsal 

 surface passes without interruption into the mesal surface of the pars canalicularis. Not all 

 the dorsal surface appears in the cranial cavity. It will be recalled that the cochlea dips under 

 the basi-occipital and basi-sphenoid, and this causes a crescentic shaped area mesally and 

 rostrally, to be shut off from the cranial cavity by that overhang of the basal plate, designated by 

 deBurlet as lamina supracochlearis. An additional portion of the dorsal surface is outside of 

 the cranial cavity, owing to the attachment of the dura to the prefacial commissure and to the 

 ridge on the surface leading from it rostro-mesad. Laterad to this ridge is an area which forms 

 the floor of the cavum supracochleare. 1 In the center of the cranial surface is a large circular 

 opening, the foramen acusticum. Laterad to this opening, divided from it by a sharp ridge, 

 is the ental opening of the facial canal. Caudal to it is a ridge which has its origin upon 

 the mesal surface of the pars canalicularis, and passes ventro-mesad 'upon the pars cochlearis, 

 diminishing in prominence as it descends. Caudal to this ridge is a large opening in the carti- 

 lage which has the shape of a figure eight, and extends to the edge of the surface. This opening 

 is the combined ductus perilymphaticus and fenestra rotunda. Their approaching separation 

 is indicated by the shape of the orifice. 



The ventral surface of the pars cochlearis is entirely hidden from view by the tympanic 

 bulla. Upon removal of this it is seen to be convex, and to give indication in the form of alter- 

 nating depressions and elevations of the cochlear turns within the capsule. Dorso-lateral there 

 is a deep depression in the surface. In this depression is the fenestra ovalis partially filled by 

 the foot plate of the stapes. Caudad to this depression the surface is raised to form the 

 promontory which intervenes between the fenestrae. The dorso-lateral edge of the depression 

 presents a ridge which marks the situation of the facial canal. Above the facial ridge, the 

 surface meets the ventral surface of the overhanging pars canalicularis. 



Pars canalicularis: - - The pars canalicularis has four surfaces, a ventral entering into the 

 tympanic cavity, a rostro-mesal, caudo-mesal, lateral, and two extremities, caudal and rostral. 

 The rostro-mesal surface joins smoothly the dorsal surface of the pars cochlearis, facing only 

 slightly more mesad than the latter. The line of demarcation can be drawn from the facial 

 canal to the foramen perilymphaticus. The surface is bounded caudally by a ridge which marks 

 the line of the crus commune of the vertical semicircular canals, and which has already been 

 described as forming a ridge upon the dorsal surface of the pars cochlearis. Upon this ridge is 

 seen a slit like opening, from which extends upward a shallow groove. This slit is the orifice 

 for the ductus endolymphaticus and the groove lodges its intracranial portion. The surface is 

 framed rostrally and laterally by the arching superior semicircular canal, and is hollowed to 

 make a well marked subarcuate fossa. 



The caudo-mesal surface is bounded rostrally by the crus commune, ventrally by the large 

 common opening of the ductus perilymphaticus and fenestra rotunda, and for the rest of its 

 boundary has the ridge marking the course of the posterior semicircular canal. It shows a slight 

 depression bordering on the crus communis, which may indicate a posterior subarcuate fossa 



1 Voit. Das Primordial cranium dps Kaninchens Anat. Hefte, Bd. 38, 1909. 



