1895.] SKULL OF OSTEOGLOSSUM FOBMOSUM. 309 



terminates anteriorly in a free extremity loosely attached by a 

 fibrous frsenum (Plate XXII. fig. 6). The upper or oral surface of 

 the " tongue " is invested by two parallel rows of small, somewhat 

 rectangular, bony plates. Over the greater part of the length of 

 the " tongue " the plates in each longitudinal row are firmly 

 connected together by interdigitating sutures, and also, in the 

 median line, with the plates of the other row ; but, as the bony 

 plates become gradually smaller and less regular in shape towards 

 the free extremity of the " tongue," the two rows become separated 

 by a median area of relatively soft skin. The oral surfaces of the 

 plates are extremely rugose, and as the sutural lines between the 

 various plates of each row form somewhat elevated ridges, it 

 follows that in addition to the general rugose character of the 

 oral surface of the " tongue " the latter is further compHcated 

 by a double series of short transversely disposed bony ridges. 

 Inferiorly, the two series of bony plates are supported by an 

 exceptionally large, forwardly directed basihyal (see Parker, I. c. 

 plate 37). In its natural position in the mouth the " tongue " is 

 situated in the median line between, but immediately below, the 

 two pterygoid bones, and at the point where these bones are 

 capable of the maximum extent of lateral movement, precisely 

 as is the case in Osteoglossum. The function of the tongue it is 

 difficult to ascertain with certainty. That it takes some part in 

 the process of mastication seems, from its structure and relations, 

 highly probable, but its precise mode of action is uncertain. If 

 the organ is capable of a rasping motion it may aid the mastication 

 of the food when the latter is firmly held by the mesial apposition 

 of the pterygoids, or, as has already been suggested in the case of 

 Osteoc/lossum, the " tongue " may co-operate by its vertical move- 

 ments with the simultaneous lateral crushing movements of the 

 pterygoids in the process of mastication. In any case the close 

 resemblance in all important details between Lepidosteus and 

 Osteoglossum as regards the structure, position, and relations 

 of the " tongue " renders it very difficult to avoid the conclusion 

 that the organ has the same physiological value in each fish, 

 and the difficulty becomes greater if we bear in mind that the 

 resemblance extends also to the essential modifications and 

 relations of the various oral bones in the two genera. 



In conclusion it may be said that Osteoglossum and Lepidosteus 

 agree in possessing an oral masticatory mechanism constructed on 

 an essentially similar plan, although it is evident from the 

 preceding description that certain relatively slight special modi- 

 fications exist in each case. Especially is the agreement shown by 

 a feature which is unique among fishes, and that is, the develop- 

 ment of a secondary movable articulation between the proximal 

 element (metapterygoid) of the mandibular arch and the skull, 

 which, in consequence, assumes a quasi-amphistylic condition. 

 Osteoglossum is, without doubt, a very generalized Teleost, and it 

 may be mentioned that there are several features in the skull 

 and in other portions of the skeleton which support this view ; 



