OF DIDELPHYS VIRGINIANA. 57 



alisplienoid and basi-occipital before, the squamosal and ex-par-occipital behind, with an 

 anterior prolongation that just touches the basisphenoid, dividing the fissurte lacerte pos- 

 teriores et medite from each other. It always remains distinct from the surrounding bones. 

 When in sifu, but little appears on an outside view ; viewed from within, it is seen to take 

 a large part in the wall of the brain-case, presenting a smooth, sub-triangular surface, in- 

 dented with a very deep, conical, cerebellar fossa, below which is the conspicuous opening 

 of the internal meatus. Most of the superficies presents a very irregular roughened sur- 

 face, closely appressed to the thickened root of the squamosal, with a venous canal 

 running across it. The part that Ues external, between squamosal and occipital, is 

 smoother, being only slightly roughened for muscular attachment. The petrosal part 

 proper appears, viewed from below, as a small, tapering piece of bone, with a swollen base, 

 this convexity expressing the dome of the " wMspering gallery" into which the interior 

 is sculptured. But the description of the internal structure of the bone rather pertains to 

 an account of the organ of hearing, than to the subject of the present memoir, and must 

 therefore be omitted. 



The tympanic is a delicate little oval ring, incomplete at part of its ch-cumference, like a 

 horse shoe. It is not bullous, and has no bony attachment with the rest of the skull. 

 It supports the auditory ossicles, one of wdiich is anchylosed with it. There are no decided 

 angles or processes ; but a noticeable groove for the membrana tympani, and another 

 representing the confluence of the ossicle ; the latter adds considerably to the tympanic 

 half-ring, strengthening the slender, spicula-like prong ; it is applied to the outer 

 periphery, terminating by a somewhat thickened, rounded extremity ; at the free end bent 

 abruptly upon itself, in the plane of the tympanum, and extending, as the handle of the 

 malleus, almost across the space to the back of the ring, as an extremely delicate spicula. 



The parietals finally coalesce with each other along the median line ; but in most 

 specimens the sagittal suture is retained. The contour of the bone is very irregular ; all 

 the sutures except the sagittal are strictly sc|uanious. The anterior margin overlaps the 

 frontal bone for a great distance ; the posterior, the super-occipital, but to less extent ; 

 conversely, the lower border is overlapped by the sphenoidal ala and the squamosal. View- 

 ing the articulated skull from the outside, the parietal seems only joined below by the 

 orbito-sphenoidal and squamosal plates ; but, on removal of the latter, the posterior 

 portion of the margin is seen to effect junction with the alisphenoid itself At least, 

 such is my impression from consideration of the parts of the sphenoidal ala, most prop- 

 erly to be regarded as ali- and orbito-sphenoidal ; for the spheno-parietal suture extends 

 to the base of the sphenoidal process that enters into the mandibular articulation. The 

 parietal bones are thin — almost diaphanous; internally they exhibit a decided hollowing, 

 whose concavity is greater than the outside convexity of the bone ; the meningeal vessels 

 impress well marked grooves along the posterior border of the vitreous table ; the fiilx cere- 

 bri a single longitudinal one along the median line. The inner table of the bone is of much 

 less extent than the outer, in consequence of the various extensive squamous sutures of the 

 latter : there is little diploe between the two ; its texture is coarse, with large interstices. 

 There were no inter-parietal bones, nor ossa Wormiana, in any of the specimens examined. 



Even in young individuals, in which the sagittal suture is perfect, there exists only a 

 trace of the frontal suture ; and this is speedily obliterated. The confluent frontals, 

 taken as one bone, present a remarkable shape. Only a small part is devoted to the pro- 



MEMOIRS B09T. SOC.NAT. HIST. VOL. II. 15 



