| OCR Text |
Show 1895.] SKULL OF OSTEOGLOSSUM FOEMOSUM. 309 terminates anteriorly in a free extremity loosely attached by a fibrous framum (Plate X X I I . fig. 6). The upper or oral surface of the " tongue " is invested by two parallel rows of small, somewhat rectangular, bony plates. Over the greater part of the length of the " tongue" the plates in each longitudinal row are firmly connected together by interdigitating sutures, and also, in the median line, with the plates of the other row; but, as the bony plates become gradually smaller and less regular in shape towards the free extremity of the " tongue," the two rows become separated by a median area of relatively soft skin. The oral surfaces of the plates are extremely rugose, and as the sutural lines between the various plates of each row form somewhat elevated ridges, it follows that in addition to the general rugose character of the oral surface of the " tongue " the latter is further complicated by a double series of short transversely disposed bony ridges. Inferiorly, the two series of bony plates are supported by an exceptionally large, forwardly directed basihyal (see Parker, I. c. plate 37). In its natural position in the mouth the " tongue " is situated in the median line between, but immediately below, the two pterygoid bones, and at the point where these bones are capable of the maximum extent of lateral movement, precisely as is the case in Osteoglossum. The function of the tongue it is difficult to ascertain with certainty. That it takes some part in the process of mastication seems, from its structure and relations, highly probable, but its precise mode of action is uncertain. If the organ is capable of a rasping motion it m a y aid the mastication of the food when the latter is firmly held by the mesial apposition of the pterygoids, or, as has already been suggested in the case of Osteoglossum, the " tongue " m a y co-operate by its vertical movements with the simultaneous lateral crushing movements of the pterygoids in the process of mastication. In any case the close resemblance in all important details between Lepidosteus and Osteoglossum as regards the structure, position, and relations of the " tongue " renders it very difficult to avoid the conclusion that the organ has the same physiological value in each fish, and the difficulty becomes greater if w e bear in mind that the resemblance extends also to the essential modifications and relations of the various oral bones in the two genera. In conclusion it m a y be said that Osteoglossum and Lepidosteus agree in possessing an oral masticatory mechanism constructed on an essentially similar plan, although it is evident from the preceding description that certain relatively slight special modifications exist in each case. Especially is the agreement shown by a feature which is unique among fishes, and that is, the development of a secondary movable articulation between the proximal element (metapterygoid) of the mandibular arch and the skull, which, in consequence, assumes a quasi-amphistylic condition. Osteoglossum is, without doubt, a very generalized Teleost, and it may be mentioned that there are several features in the skull and in other portions of the skeleton which support this view |
