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Show 1895.] SKULL OF OSTEOGLOSSUM FORMOSUM. 307 articulation of the metapterygoid with the skull, the result of which is to give an altogether exceptional precision to such movements in accordance with the requirements of the highly specialized oral masticatory apparatus of this fish. I am unable to say how far the possession of the mechanism is peculiar to Osteoglossum formosum among the Osteoglossidae. It would certainly be interesting to ascertain whether it is restricted to this species, or is shared by either, or both, of the two remaining species of the genus, viz. 0. bicirrhosum and 0. leichardti • and also whether the mechanism is present in the sole remaining genera of the family, Heterotis and Arapaima. It has been remarked that the peculiar metapterygo-parasphe-noidal articulation of Osteoglossum formosum has no parallel in any other Teleostean fish, but it is nevertheless interesting to remark that an essentially similar mechanism is to be found in Lepidosteus osseus. In this Ganoid the basicranial articular surface for the metapterygoid is formed in part by a lateral outgrowth from the parasphenoid, and partly also by a descending process from the adjacent portion of the prootic, the former process forming the inner, and the latter the outer half of a transversely-elongated condyle provided with smooth, rounded, anterior and ventral surfaces. In his valuable paper on the " Development of the Skull in Lepidosteus osseus " (Phil. Trans. Roy. Soc. 1882) the late Professor Kitchen Parker refers to these condyles under the name of " basipterygoid processes," and describes them as being ossified by the alisphenoids. It is difficult, however, to see how this can be the case, at any rate from an examination of the adult skull. Tbe processes in question certainly seem to be formed to an equal extent by the parasphenoids and the prootics, and are widely separated from the alisphenoids. The connection between the proximal elements of the hyoid arch and the corresponding bones of the mandibular arch is neither so intimate nor so rigid as in Osteoglossum; but, on the other hand, the metapterygoid, quadrate, pterygoid, and mesopterygoid bones are very firmly and rigidly united together by overlapping sutures. The metapterygoid (see Parker, loc. cit. plate 37. fig. 4, mt.pg.) is inclined obliquely downwards and forwards, so that the long axis of the bone makes an acute angle with the anterior section of the skull. The upper or cranial extremity of the bone is furnished with a transversely disposed concave surface for articulation with the condyle provided for it by the parasphenoid and prootic. The palato-pterygoid series consists of a relatively small mesopterygoid, which overlaps the dorsal borders of the pterygoid and quadrate bones ; and an exceptionally large pterygoid, firmly applied posteriorly to the inner surfaces of both the metapterygoid and the quadrate, and gradually tapering to a thin, flexible, splint-like anterior portion. The latter part of the pterygoid (see Parker, I. c. plate 37. fig. 3, pg.) forms part of the slender rostral portion of the skull, and is situated between the parasphenoid (pa.s.) and vomers (v.) mesially, the superficial palatine (pa.1) and the segmented maxilla (mx.1) 20* |