306 puor. T. w, BBiDGE ON THD [Apr. 2, 



and its functional relations to other parts of the mechanism, must 

 not, however, be left out of consideration. This organ (fig. 5) is 

 an elongated band-like structure, somewhat broader behind than 

 in front, and consists of a continuous plate of tooth-bone confluent 

 below with the upper surfaces of the large basihyal and the two 

 most anterior of the basibranchial elements. The tooth-bone carries 

 over the whole extent of its upper or oral siu-face an extensive 

 series of closely-set teeth, the largest of which are arranged in 

 three parallel longitudinal rows — a lateral series (l.t.) along each 

 outer margin of the " tongue," and a mesial row (m.t.) along the 

 centre. The lateral teeth, although somewhat smaller, are very 

 similar in shape and disposition to the linear series in each 

 mesopterygoid, and, like the latter, diminish in size from behind 

 forwards ; the central teeth, on the contrary, are smaller in size, 

 obtusely conical in shape, and much less regularly arranged. 

 Between the three principal series of " lingual " teeth the surface 

 of the " tongue '" has a granulated appearance from the presence 

 of a number of minute, conical, closely-set teeth. In its natural 

 position in the mouth the linear series of lateral teeth are 

 vertically opposable to the corresponding series of mesopterygoid 

 teeth, while the mesial series lie between and below the latter. It 

 is probable, therefore, that all three series of teeth (viz, the 

 mesopterygoid, the parasphenoidal, and the "lingual") co-operate 

 in effecting the mastication of the food — the mesopterygoid teeth 

 by means of their lateral motion in the horizontal plane, and the 

 lingual teeth through their vertical movements, or possibly, but 

 improbably, by a kind of rasping action. 



The capacity for lateral movement on the part of certain of the 

 oral bones is by no means peculiar to Osteoglossum. In the Pike 

 (Esox), for example, and probably also in many other Teleosts, 

 there can be no doubt that the proximal elements of the hyoid 

 and mandibular arches and the various bones of the palato- 

 pterygoid series are capable of similar movements as a more or 

 less rigid whole, and that, in consequence, the jaw-system is 

 similarly capable of lateral expansion and contraction. The 

 capacity of the bones in question for lateral movement is no doubt 

 of great service in widening the gape and thereby adapting the 

 jaws for seizing relatively large prey, or it may be of considerable 

 advantage in enabling the fish to firmly retain its possibly 

 struggling prey in the oral cavity prior to the act of swallowing, 

 or even in effectually aiding deglutition itself ; but, at the same 

 time, it is extremely doubtful if anything of the nature of oral 

 mastication can be effected by these means. On the other hand, 

 in none of the ordinary Teleosts is there any articulation between 

 the metapterygoid and the base of the skidl, and whatever lateral 

 mobility the bones under discussion possess is entirely due to the 

 nature of the hyomandibular-pterotic and the ethmo-palatine 

 articulations. The special peculiarity of Osteoglossum lies in the 

 fact that, while the jaws are capable of the same kind of lateral 

 movement as in the Pike, there is, in addition, a secondary 



