270 BK. W. G. HTDEWOOD ON THE 



Arapaima, but not in Heterotis ; the eye-muscle canal does not 

 open posteriorly. 



In all three the post-temporal has well- developed epiotic and 

 opisthotic limbs ; the orbit is surrounded by five or six bones, 

 of which the postorbitals are the largest. The gape is bounded 

 above by the premaxiila and maxilla ; teeth are borue by the 

 premaxilla, maxilla, and deutary ; there is no surmaxilla. The 

 angular bone is distinct from the articular ; it takes part in the 

 formation of the articular facet for the head of the quadrate iu 

 Arapaima and Seterotis, but not in Osteoglossum. The endosteal 

 articular is separable from the ectosteal articular iu Arapaima, 

 but not in Osteoglossum and Heterotis, A sesamoid articular is 

 present in Osteoglossum, and a small one in Heterotis, but it is 

 apparently wanting in Arapaima. The symplectic exhibits a 

 tendency to spread over the adjacent bones in Arapaima and 

 Osteoglossum, but the tendency is less apparent in Heterotis. 

 In Heterotis there is a distinct palatine bone, but in Osteoglossum 

 and Arapaima the palatine is not separate from the ectopterygoid. 

 Teeth are borne by the entopterygoid in Heterotis, and by the 

 entopterygoid and the combined palatine and ectopterygoid in 

 Osteoglossum and Arapaima. 



The subopercular is small in Osteoglossum and Arapaima, and 

 very small in Heterotis. The branchiostegal rays number 8 on 

 each side iu Heterotis, 10 or 11 in Arapaima, 8 in Osteoglossum, 

 licirrhosum (Boulenger, Ann. Mag. Nat. Hist. viii. 1901, p. 514, 

 says 10), 10-13 in Osteoglossum Leichardti (Boulenger says 15- 

 17), and 14 or 15 in Osteoglossum formosum (Boulenger says 

 15-17). 



The hypohyal is single on each side. A large dentigerous 

 lingual bone is present ia Osteoglossum and Arapaima ; in 

 Heterotis it is only of moderate size. In Osteoglossum and 

 Heterotis there is a downwardly directed process of the mesial 

 end of the second hypobranchial, as iu Mormyroid fishes ; in 

 both of these forms the glossohyal cartilage is unossified, and in 

 Heterotis the first basibranchial also, a fact to which Cope 

 attaches some importance (Trans. Amer. Phil. Soc. n. s. xiv. 1871, 

 p. 455). The first pharyugobranchial is unossified and the 

 spicular bone is absent. Cope (l. c.) states that the " superior 

 pharyngeals " are three in number on each side in Osteoglossum, 

 and two in Heterotis; the branchial skeletons of the specimens 

 at my disposal are not sulEciently perfect to enable me to 



