268 DE. W. G. RIDEWOOD ON THE 



towards the hyomaudibular, metapterygoid, quadrate, and pre- 

 opercular. The thickened axis, however, can be recognized, and 

 this makes an angle of about 50 degrees with the horizontal 

 plane. 



Projecting from the back of the quadrate is a considerable 

 lamina of bone (apparently membrane-boue) which flanks the 

 outer surface of the lower part of the symplectic and lies just 

 above the horizontal limb of the preopercular. The ectoptergoid 

 is not bent, and is indistinguishably united with the palatine. 

 In this feature, and in the absence of any endosteal part of the 

 palatine, Arapaiona resembles Osteoglossum. The buccal surfaces 

 of the pterygoid and palatine bones are almost completely covered 

 with densely crowded small teeth. The lateral process of the 

 parasphenoid, already referred to, fits into an elliptical foramen 

 bounded above and behind by the hyomandibular, below by the 

 hyomandibuJar and metaptergoid, and in front by the entoptery- 

 goid. The synovial articulation is with the entopterygoid. 



Opercular Series (fig. 15). — The opercular bone is of average 

 proportions ; it is nearly semicircular in outline, and is sculptured 

 over the whole of its outer surface except near the upper and 

 anterior edges. The subopereular is small, triangular, and not 

 sculptured. There are ten or eleven branchiostegal rays, of 

 which five or sis are attached to the ceratohyal, and the remainder 

 to the epihyal. They constitute an evenly graduated series ; the 

 front ones have the form of slender curved rods, the hinder 

 ones are larger, and slightly expanded and lamellate. Agassiz 

 (Spix, Pise. Bras. 1829, Anat. Plate B, fig. 5) shows ten i-ays ; 

 Valenciennes (Hist. JSTat. Poiss. xix. 1846, p. 438), Griinther 

 (Brit. Mus. Cat. Pishes, vii. 1868, p. 376), and Boulenger (Ann. 

 Mag. Nat. Hist. [7] viii. 1901, p. 515) are agreed in putting the 

 number at sixteen. 



Syohrancliial Series. — In neither of the specimens available 

 is the hyobranchial skeleton complete. The interhyal is ossified. 

 There is a single hypohyal on each side, and this is probably the 

 equivalent of the lower of the two that are present in so many 

 other Teleostean fishes. The first, second, and third basi- 

 branchials are overlaid by a readily removable dentigerous plate 

 of the form of a greatly elongated ellipse, the teeth of whicli are 

 densely crowded and similar to those of the parasphenoid and 

 pterygoid bones. Behind this is a small circular dentigerous 

 bone of similar character. 



