1895.] SJCULL OP OSTEOGLOSSUil i'OfiMOSUM. 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 hyoma,ndibular 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 meta- 

 pterygoids and the parasphenoid wUl 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 Osteoc/lossuin, 

 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 we 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 may 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, 



Peoc. Zool. Soc— 1895, No. XX. 20 



