OCR Text |
Show 308 PEOF. T. W. BBIDGE ON THE [Apr. 2, externally, and the frontal (/.) and ethmo-nasal (et.n.) bones above. The bones forming the lateral portions of the rostrum or snout-that is, the pterygoid, the superficial palatine, and the divided maxilla-are firmly connected together, but their connection with the mesial elements of the rostrum-the parasphenoid, vomers, and ethmo-nasals-is loose and ligamentous. The usual ethmo-palatine articulation is apparently altogether wanting in Lepidosteus; but the necessary lateral mobility of the palato-pterygoid series in front is secured, (i.) by the flexibility of the thin anterior portion of the pterygoid, and (ii.) by the loose ligamentous connection which has been described as existing between the palato-pterygoid series, including the maxillae, and the mesial bones of the beak. The place of the normal ethmo-palatine joint is apparently taken by a secondary articulation furnished by a smooth oval surface on the inner side of a preorbital process of the frontal and a similar facet on the superior border of the pterygoid. It m a y be concluded, therefore, that, as in Osteoglossum, the palato-pterygoid series of bones are capable of more or less extensive lateral movement as a rigid whole on the metapterygo-parasphenoidal and the pterygo-frontal articulations, and, further, that such lateral movements are accompanied by the alternate approximation and separation of the two pterygoid bones in the mesial line of the oral cavity. As regards the character of the teeth supported by the oral bones, Lepidosteus differs considerably from Osteoglossum. Apart from the acutely pointed teeth in the maxilla?, there is a series of similar but much smaller teeth in the superficial palatines, and also a number of closely-set denticles on the vomers and on a limited area of the oral surface of each pterygoid. These teeth, however, can only be of service in the vertical movements of the ordinary jaws, and even in the case of the pteiygoid teeth it is obvious, from their position, size, and mode of implantation, that they can have no functional significance in connection with the lateral mobility of their supporting bones. It is possible that, as in Esox, the lateral mobility of the palato-pterygoid bones and the maxillae is simply to admit of the lateral expansion of the jaws when seizing relatively large prey ; but although this m a y be one of the advantages which the fish derives from this mechanism, the probability that the partially swallowed prey may be subjected to a process of crushing in the oral cavity by the lateral movements of the pterygoids must, nevertheless, not be lost sight of. It may also be remarked that the latter suggestion is strongly supported by the fact that the superior or inner edges of the two pterygoid bones are capable of being brought into actual contact in the median fine of the mouth-cavity for a considerable portion of their length. There is another interesting feature in which Lepidosteus resembles Osteoglossum, and that is the structure of the " tongue." In the Ganoid the floor of the mouth is elevated to form a well-marked, elongated, and somewhat strap-shaped " tongue," which |