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Show 398 PROF. HUXLEY ON THE REPRESENTATIVES OF [.May 27, outer end of the tympanic cavity ; when it is removed, the proximal end of the cornu of the hyoid is seen to expand, and becomes converted into a broad plate of cartilage, the curved margin of which gives rise to the " scroll." Internally the plate is continued into the stem of the stapes, and speedily becomes ossified (fig. 4). There can be no doubt, therefore, that it corresponds with the extrastapedial cartilage of the Crocodile. What answers to the axehead-shaped suprastapedial cartilage of the Crocodile is the upper process of the cartilaginous part of the stapes (S. St), which, however, passes into the extrastapedial cartilage externally and above, so as to enclose the foramen, a (fig. 4). On the left side, the suprastapedial process was fibrous at the point b (fig. 4). Superiorly, the suprastapedial cartilage (S. St) is directly continued into the cartilaginous termination of the parotic process of the skull (Pc.C), in which granular osseous matter is deposited. Thus the suprastapedial cartilage turns out to be nothing more than the proximal end of the hyoidean arch, while the stapes and its appendages are exclusively related to this arch, and have nothing whatever to do with the mandibular arch. With respect to the stapes (or columella auris) in Birds, I may remark that Prof. Peters makes no reference to the careful investigations of Platner (' Das Quadratbein der Vogel'), who takes the same view of the homology of that structure as himself. Platner observes (/. c. p. 16) : - " The functions of the parts which lie within the tympanic cavity of birds almost all bear upon tbe tightening or relaxation of the tympanic membrane. If we consider the apparatus directed to this end in the tympanic cavity of Birds, we find :-(1) the stapes with the attached cartilage, which represents the other auditory ossicles in an aborted condition; (2) the tendon of the tensor tympani, which arises from the lower part of the occiput and is attached partly to the stapes and partly to the tympanic membrane ; (3) an elastic ligament, which I have found in all the birds which I have examined, and which passes from the base of the cartilage which is attached to the stapes, on the side which is opposite to the insertion of the tensor tympani (i. e. from the front aspect), up to the tympanal articulation (Paukenbbhlengelenk) of the os quadratum, where it unites with the mucous membrane which covers the tympanic cavity internally." The disposition of the parts described by Platner, in the Common Fowl, is shown in the accompanying figure (fig. 5). As in the Crocodile, the stem of the stapes is ossified and ends in a triradiate cartilaginous expansion. When the skull is horizontal one ray of the expansion is represented by a horizontal triangular plate, the outer edge of which is fixed to the tympanic membrane. It represents the extrastapedial cartilage of the Crocodile, but differs therefrom in being perforated by a large hole. Where the stem of the stapes passes into this plate, it gives off a short vertical process upwards and a long slender curved process downwards. Both these lie free in the tympanic cavity. From the root of the upper process an elastic ligament arises and passes forwards to the |