484 Mft, R. J. POCOCK ON TSE 



Museum shows that the cavity is broken up by trabeculse into 

 spaces which extend into the mastoid. It is not possible, how- 

 ever, to ascertain from this specimen whether the cavity, before 

 fracture, was divided by a- partition into two chambers as in 

 Grisonella or not. 



In Mephitis, Spilogale, and Conepatus the bulla is still more 

 peculiar. As in Grisonella, the tympanic ring is fused with the 

 roof of the bulla and the cavity of the latter is a hollow space, 

 with at most a few septa or bony ridges, which is closed just 

 behind the periotic by a thickish wall of bone lying between the 

 stylomastoid foramen and the foramen lacerum posticiton ; bu.t 

 opposite the outer side of the periotic there is a tolerably large 

 oval orifice by which the cavity of the bulla communicates with 

 a large hollow space occupying the expanded area of the skull 

 between the auditory orifice and a point on the occipital crest 

 above the paroccipital process. Behind the bulla externally there 

 is a tolerably long, flat area running back to the paroccipital 

 process, which, as in Lutra, is placed some distance behind the 

 bulla. 



In Mephitis and Spilogale the cavity in the periotic runs inwards 

 by means of a narrow passage up to the foramen lacerum pos- 

 ticum, and the posterior margin of the orifice, by which the cavity 

 communicates with the bulla, is produced into a crescentically 

 curved ridge of bone. This ridge and the naiTow passage running 

 to the foi-amen lacerum posticiom are absent in Co7iepatus proteus, 

 the walls of the cavity being undiS'erentiated. 



In the British Museum there is a skull of Mydaus with a 

 broken bulla, which seems to show that in essential points the 

 bulla of this genus resembles that of Mephitis. The cavity of the 

 bulla, for instance, comes to an end posteriorly just behind the 

 petrous portion of the periotic, and on the outer side of this bone 

 there is an orifice by means of which the cavity of the bulla 

 communicates with the hollowed mastoid portion of the periotic. 



The following table summarises the principal variations in the 

 structure of the bulla : — 



a. Cavity of bulla closed behind, not communicating with a hollow 



space in the external portion of the periotic, the latter being 



solid or nierel}' permeated with air-cells. 



h. Bulla distinctly divided into two subequal chambers by a 



bony partition running obliquely inwards and forwards from 



posterior end of tympanic ring to carotid foramen, the two 



chambers communicating by means of a notch in the lower 



edge of the partition behind posterior end of petrous portion 



of periotic Selictis. 



h' . At most an incomplete partition passing transversely from 

 posterior portion of tj'mpanic ring to i\\& foramen lacerum 

 posticwm. 

 c. Walls of bulla exceedingly thict, permeated everywhere 



with air-cells. No partition Musfela, Putorms, lyo7'meIa'\. 



c'. Walls of bulla thin, without noticeable air-cells, 



d. Tympanic ring closely applied to roof of bulla, the pos- 

 terior end of which is far in advance of the paroccipital. Ijutra, 



