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Show 1869.] THE MALLEUS AND THE INCUS OF THE MAMMALIA. 405 passes outwards, and is inserted into the upper aspect of that kneelike process of the malleus which is fixed to the tympanic membrane. The cartilaginous " styloid " end of the hyoidean arch is fixed into the wall of the outer and posterior end of the tympanic cavity, very near the incus and stapes ; but I can find neither a stapedius muscle, nor any ligament representing it. It will be observed that the proximal end of the skeleton of the first visceral arch (whether it be osseous, cartilaginous, or fibrous), like that of the second, remains attached to one and the same part of the skull, viz. the outer and upper wall of the periotic mass, external to the vestibular sac, throughout the Mammalia and the Sauropsida. In Mammals the proximal skeletal elements of the arches (malleus and incus) are very generally equal, or the incus may be the smaller. In the Sauropsida, the suprastapedial (=incus) is always smaller than the quadratum ( = malleus). In Teleostean and Ganoid fishes, and in the Sharks, the general relations of the two arches remain unchanged, but their proportions are reversed. The only vertebrated animals in which a portion of the first visceral cleft remains open throughout life are some Ganoidei and most Elasmobranchii, in which, according to Wyman's observations, the spiracle is the remains of that cleft. It follows that any skeletal part which bounds the spiracle posteriorly cannot belong to the first, or mandibular, visceral arch, but must appertain to the second, or hyoidean arch. Now the suspensorial cartilage of the Elasmobranchs occupies this position. Its proximal end is attached to the outer wall of the auditory capsule ; its distal end bears the proper hyoidean arch. Thus it answers exactly to the upper end of the second cartilaginous visceral arch, and therefore must contain the homologue of the incus. But the suspensorial cartilage of the Elasmobranchs is undoubtedly the homologue of the hyomandibular bone and symplectic of the osseous Ganoidei and of the Teleostei-which, therefore, must, in part or wholly, answer to the incus. Where, then, is the homologue of the proximal end of the skeleton of the first visceral arch of the fish, if the hyomandibular belongs altogether to the second? I find it in that prolongation of the quadrate cartilage of the Teleostean which ascends in front of the hyomandibular, and is at first quite free from it, but afterwards becomes surrounded and replaced by the metapterygoid, which eventually helps to bind it to the hyomandibular. Thus the puzzling division between the mandibular and the hyoidean parts of the suspensorial apparatus in a fish becomes intelligible as the result of their primarily separate development. In the osseous fishes the proximal end of the mandibular arch is arrested in its development and loses its direct connexion with the skull; but in the Sharks the ascending portion of the quadrate atrophies altogether, or is represented merely by pre-spiracular cartilages ; and the quadrate itself forms only the posterior termination of the palato-quadrate arch, or so-called upper jaw. P R O C ZOOL. Soc-1869, No. XXVII. |