1895.] SKULL OF OSTEOGLOSSU5I fobmosthi. 307 



articulation of the metapterygoid \^ith the skull, the result of 

 which is to give an altogether exceptional precision to such 

 movements in accordance with the requirements of the highly 

 speciaUzed oral masticatory apparatus of this fish. 



I am unable to say how far the possession of the mechanism is 

 peculiar to Osteoglossum formosum among the Osteoglossidse. It 

 would certainly be interesting to ascertain whether it is restricted 

 to this species, or is shared by either, or both, of the two remaining 

 species of the genus, viz. 0. bicirrhosum and 0. leichardti ; and 

 also whether the mechanism is present in the sole remaining 

 genera of the family, Heterods and Arapaima. 



It has been remarked that the pecuHar metapterygo-parasphe- 

 noidal articulation of Osteoglossum formosum has no parallel in any 

 other Teleosteau fish, but it is nevertheless interesting to remark 

 that an essentially similar mechanism is to be found in Lepidosieus 

 osseiis. In this Ganoid the basicranial articular surface for the 

 metapterygoid is formed in part by a lateral outgrowth from the 

 parasphenoid, and partly also by a descending process from the 

 adjacent portion of the prootic, the former process forming the 

 inner, and the latter the outer half of a transversely-elongated 

 condyle provided with smooth, rounded, anterior and ventral 

 surfaces. In his valuable paper on the " Development of the 

 Skull in Leindosteus osseus "' (Phil. Trans. Eoy. Soc. 1882) the late 

 Professor Kitchen Parker refers to these condyles under the name 

 of " basipterygoid processes," and describes them as being ossified 

 by the alisphenoids. It is difficult, however, to see how this can 

 be the case, at any rate from an examination of the adult skull. 

 The processes in question certainly seem to be formed to an equal 

 extent by the parasphenoids and the prootics, and are Avidely 

 separated from the alisphenoids. The connection between the 

 proximal elements of the hyoid arch and the corresponding bones 

 of the mandibular arch is neither so intimate nor so rigid as in 

 Osteoglossum ; but, on the other hand, the metapterygoid, quadrate, 

 pterygoid, and mesopterygoid bones are very firmly and rigidly 

 united together by overlapping sutures. The metapterygoid (see 

 Parker, loc. clt. plate 37. fig. 4, mt.jyg.) is inclined obliquely 

 downwards and forwards, so that the long axis of the bone makes 

 an acute angle with the anterior section of the skull. The upper 

 or cranial extremity of the bone is furnished with a transversely 

 disposed concave surface for articulation with the condyle provided 

 for it by the parasphenoid and prootic. The palato-pterygoid 

 series consists of a relatively small mesopterygoid, which overlaps 

 the dorsal borders of the pterygoid and quadrate bones ; and an 

 exceptionally large pterygoid, firmly applied posteriorly to the 

 inner surfaces of both the metapterygoid and the quadrate, and 

 gradually tapering to a thin, flexible, sphnt-Uke anterior portion. 

 The latter part of the pterygoid (see Parker, I. c. plate 37. fig. 3, 

 jpg.) forms part of the slender rostral portion of the skull, and is 

 situated between the parasphenoid (jpa.s.) and vomers (v.) mesially, 

 the superficial palatine (jxt.^) and the segmented maxilla {mx.^) 



20* 



