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Show 406 REPRESENTATIVES OF THE MALLEUS AND THE INCUS. [May 27, Iii the Chimcerce, Dipnoi, and all Amphibia, the proximal ends of the cartilaginous first and second visceral arches become united together, at an early period, into a common plate, in which the malleal and incudal elements are not separately distinguishable. In the Chimcerce, Dipnoi, and the lower Amphibia they remain in this condition throughout life ; but in the higher Amphibia changes of a most remarkable kind take place, of which I do not now propose to speak, as m y friend Mr. Parker is engaged in working out that part of the subject. I have endeavoured to give visual expression to m y interpretation of the modifications of the proximal ends of the mandibular and hyoidean arches in the series of diagrams A, B, C, D, E, F. displayed in fig. 8. In all these the mandibular arch is shaded with vertical lines, while the hyoidean arch is left unshaded or nearly so. The letters have the same signification throughout. In the mandibular arch:-Qu. Quadrate. Mn. Mandible or Meckel's cartilage. In the hyoidean arch :-a. Extrastapedial. b. Suprastapedial. Stp. Stapes. St. Stylo-hyal. Hy. The ventral moiety of the hyoidean cornu. A. Sphenodon. B. Crocodile. C. Bird. D. Mammal. E. Teleostean fish. F. Shark. In D, ob stands for the orbiculetre; in E, Mpt is the metaptervgoid, and By the symplectic. |