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Show 254 ON THE STRUCTURE OF THE SKULL. it. The tympanic, however, fonns only the outer pa~t of the posterior wall of the tympanum, the inner and _ postenor vval~s of that cavity being furnished by a downward process of the basioccipital, while its inner and ante~ior wal~ is fm·1ned partly by the pterygoid and partly by the ahsphen~Id. These tw~ bones enclose a great air-cell, which con1municates freely w1th the tympanic cavity behind. In front, it is dosed by a thin bony partition, which separates it from a second large air-chamber, enclosed, partly by the alisphenoid and pterygoid, and partly by the palatine. In the genus Manis there is a large bulla, forn1ed altogether by the tympanic, which, in moderately young skulls, at any rate, is not anehylosed with the adjacent bones. The squamosal is an immense bone, extending from the exoccipital to the orbito-sphenoid, and entering into the lateral walls of the skull for that extent. Its posterior part, dilated and convex outwards, contains a large air-cell, which opens into the roof of the tympanu]n by a wide aperture. The plane of the periotic is nearly horizontal. It is a relatively small bone, and only a small part of it appears on the base of the skull, behind the tympanic bulla, the squamosal completely hiding it externally. Of the Armadillos, some, like Euphractus, have a tyn1panic bulla of the ordinary construction, with, occasionally, a very long external auditory meatus; while others, such as the ninebanded Armadillos (Praopus of Burmeister), have a mere hoop of bone open above, almost as rudimentary as that of Echidna. Or, if we turn to the perissodactyle Ungulata:-· In the Rhinoceros, the periotic and tyn1panic early anchylose together, but remain distinct from the surrounding bones, the compound tympano-periotic being only wedged in between the squamosal, ex-occipital, and other · adjacent cranial bones, in such a manner that it cannot fall out. The "pars mastoidea " is completely hidden, externally, by the union of the squamosal and the paramastoid process of the ex-occipital over it. The region itself, however, is very well developed, and is continuous, internally and below, with a very strong, conical, somewhat curved, styloid process, to the flattened, free base of which the hyoidean apparatus is attached. THE SKULLS OF MAMMALIA. 255 rrhe ty1npanic element is very singularly formed. It has the shape of a very irregular hoop, open above and behind, and much thicker at its anterior superior than at its posterior sup~ rior end. . Tl.le ~ormer, ~rregular and prismatic, is anchylosecl ~VIth the pe~·1ot~c, JUSt beh:n_d. and above the auditory labyrinth; It then spbts Into two diVISIOlls, an anterior and inner and a posterior and outer. The anterior, acquiring a thick and spongy texture, curves round to form the front part of the wall of the tympanu1n, and then ends in a free, backwardly-directed apex, without becoming in any way connected with the periotic, or with the posterior division. The latter, much thinner and denser, c~rves downwards and backwards in the same way, and also remains perfectly free, but its hinder end is prolonged into a flat process, which bends for a short wav round the base of the styloid process. The outer wall of the" tympanum is therefore very incomplete in the dry skull, opening forwards and downwards, first, by the fissure between the anterior branch of the tympanic and the periotic; and, secondly, by the cleft between the two divisions of the tympanic. Posteriorly, there is a large irregular aperture between the hi~der end of the anterior branch of the tympanic and the periotic. Externally, there is no bony auditory meatus-or rather the merest rudiment of one. The Horse presents a very different structure. There is a tympano-periotic bone which is wedged in between the squanlosal and adjacent bones, and not anchylosed therewith; but the pars mastoidea appears largely on the outside of the skull between the post-auditory process of the squamosal and the parama~toid, and the tympanic elmnent consists of a complete bulla: With a long external auditory meatus. The styloid process 1s almost con1pletely infolded by a vaginal process furnished by the auditory Ineatus, and the tympanic is altogether anchylosed to the periotic, posteriorly. No Tapir's skull which I have examined has presented any trace of an ossified tympanic bone.* In the Horse, most Primates, Carnivores and Rodents, the * Ac?~rding to Cuvier, "I/os de la caisse ne paroit jamais bien se souder avec les os vo1sms et tomb€ aisement, comme dans l'herisson, le sarigue," &c. |