256 PEOFESSOE OWEN ON CNEanOENIS. 



augment the tympanic cavity, with which such " bulla " communicates by an aperture 

 (PI. XXXV. fig. 1, t a) below the inner articular facet for the tympanic. The tympanic 

 cavity opens upon the basis cranii by a wider aperture (ib. fig. 4, th), directed outward 

 and forward as well as downward, of a transversely elliptical form, which seems to be 

 bisected at a higher level by the process of bone (from the alisphenoid), forming the 

 inner articular cavity for the .inner division or condyle of the head of the tympanic. 

 A vacuity divides this from the outer articular cavity, which looks inward and a little 

 downward. 



The prebasal aperture of the tympanic cavity (PI. XXXV. fig. 4, t h) is bounded 

 behind by the bar of bone extending, as in Cereopsis and Ajjtornis ', from the side 

 of the basisphenoid (ib. figs. 1 & 4, 6») to the mastoid process (ib. fig. 1, 8'). This bar 

 bounds the fore part of the lateral opening of the tympanic cavity (ib. figs. 1 & 4, # /). 

 The inner wall of this cavity is perforated by two openings leading to the pneumatic 

 cancellous structure of the cranial walls. The paroccipital forms the hinder wall of the 

 tympanic cavity, and is continuous by the thin plate forming the lower part of the 

 inner wall of the cavity with the basisphenoidal pier of the vertical " sphenomastoid " 

 bar or arch (fig. 1, S'-S'). 



The sphenoidal tympanic bulla is homologous with that in the Marsupial genus 

 Peragalea^. Anterior to it opens the foramen ovale (figs. 1 «& 4, e), divided, on 

 one or both sides, by a slender bar between the issue of the motory and larger 

 sensory parts of the third division of the trigeminal nerve. Five lines in advance 

 and mesiad of this is the " foramen rotundum ;" and two lines in advance of this is 

 the larger elliptic foramen for the optic nerve (ib. fig. 1, lo) and first division of the 

 trigeminal. 



On the inner surface of the cranium the petrosal is impressed above the cribriform 

 depression, representing the " foramen auditorium internum " by a very deep vertically 

 elliptical fossa, answering to the "appendicular fossa" in that part of the epen- 

 cephalic chamber of certain Marsupialia '. The side walls of the epencephalic com- 

 partment are from 9 lines to 5 lines in thickness, and are occupied by air-cells ; the 

 walls of the prosencephalic com])artment are thinner, but still with a pneumatic 

 diploe. 



' In the description of the sphenomastoid part of the skull of Aptomis defossor it is stated : — 



" The articulation is close and deep, whereby, with a peculiar suspensory structure, the tympanic is retained 

 OQ the right side of the present skull, where the surrounding parts of the cavity are entire." The structure is 

 described as follows : — " This process " (the mesomastoid) " has contracted a filamentary bony union with the 

 expanded base of the alisphenoid, the filament passing behind the neck of the tympanic, helping to suspend and 

 maintain it in situ." — Trans. Zool. Soc. v. (1870) p. 356, pi. 40. fig. 1,8'. 



' Perameles lagotis. Art. Marsupialia, Cycl. of Anatomy, vol. iii. p. 274, fig. 96. 



' " The petrous bone in the Kangaroo, Koala, and Phalanger, ia impressed above the ' meatus auditorius in- 

 ternus ' by a deep, smooth, round pit, which lodges the lateral appendage of the cerebellum." — Art. Marsupialia, 

 ut supra, p. 274. This is the " appendicular fossa ; " the " floccus " of Ecil is a different part of the cerebellum. 



