OCR Text |
Show 1895.] SKULL OF OSTEOGLOSSUM i'OEMOSUM. 305 opposite side of the oral cavity, instead of being at right angles to the surface of the mesopterygoid like the remaining teeth which this bone supports. From this description it is apparent:- i. That the various bones which form the proximal half of the mandibular arch and its palato-pterygoid bar are firmly rigidly connected with one another, and also with the corresponding elements of the hyoid arch-that is, with the hyomandibular and symplectic bones. ii. That, in consequence of the articular connection of the hyomandibular with the periotic capsule, and the metapterygoids with the articular processes of the parasphenoid, combined with the looseness of the ethmo-palatine connection, the whole series of bones on each side are capable of a more or less extensive inward and outward movement, accompanied at the same time by the lateral contraction or expansion of the oral cavity. It is also clear that while the hyomandibular articulation with the skull and the nature of the ethmo-palatine connection will give the necessary mobility to the rigidly connected series of bones considered as a whole, the sliding character of the joint between the metapterygoids and the parasphenoid will give precision to such movements by strictly limiting them to the alternate approximation and separation of the bones of opposite sides of the head. iii. That, as the result of such movements, the linear series of obliquely set teeth in the two mesopterygoids become opposable in the median line of the oral cavity, and, in conjunction with the mesial teeth in the parasphenoid, form part of an additional oral masticatory mechanism, distinct from the usual mechanism which is furnished by the upper and lower jaws and their teeth. I have no knowledge of the character of the food of Osteoglossum, but, whatever may be its nature, it is evident that the food is subject to some kind of mastication in the oral cavity by the two parallel series of mesopterygoid teeth after its seizure in the first instance by the ordinary jaws. From this point of view there seems to be a fairly close analogy between Osteoglossum and the Crayfish, inasmuch as the relative functions of the mandibles and the gastric mill in the latter would seem to be performed by the ordinary jaws, and by the mesopterygoid and parasphenoidal teeth respectively in the former. In fact the analogy becomes even more precise if w e consider that the relative position and relations of the mesopterygoid and parasphenoidal teeth in Osteoglossum are essentially similar to those of the zygocardiac and urocardiac teeth in the gastric mill of the Crustacean. It m a y be pointed out, however, that the two series of mesopterygoid teeth can scarcely come into actual contact in the median line of the oral cavity, but, on the other hand, it is certain that they can be brought so close together as readily to effect the crushing or more or less complete mastication of food which has been taken into the mouth. In discussing the functions of the oral masticatory mechanism of Osteoglossum the singular dentigerous "tongue" of this fish, PROC ZOOL. Soc-1895, No. XX. 20 |