168 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



the mouth. The skull top is short and fairly broad, with a high projecting supraoccipital 

 crest, which is flattened and thick on top. The broad supraoccipital base has grown 

 forward, thrusting apart the parietals, or overgrowing them, in the rear; but they still 

 retain a short contact in the mid-line in front of the supraoccipital, separating the latter 

 from the broad frontals. In Ridewood's specimen but not in mine the large "supra- 

 temporal" or scale bone covered over an oval tract of cartilage corresponding perhaps 

 with the pre-epiotic fossa of clupeids, and perhaps related morphologically to the lateral 

 cranial foramen of mormyrids. At the side of the base of the cranium and below the level 

 of the horizontal ridge on the pterotic and opisthotic is a great vesicle of the swim-bladder. 

 "Its outer wall," writes Ridewood, "is composed of fibrous tissue, which is attached to the 

 cranium along the line marked with dots in Fig. 20. Its inner wall is formed by the 

 exoccipital and basioccipital and its anterior wall is formed by a vertical lamina of the 

 pro-otic. Between the exoccipital, basioccipital and pro-otic is a fairly large auditory 

 fenestra, opening into the perilymphatic cavity and traversed vertically by the pro-otic 

 lamina just mentioned." Ridewood points out that the auditory fenestra is a clupeoid 

 feature and that its occurrence in Hyodon is of some interest. 



The circumorbital plates are fairly normal but the interopercular is concealed in the 

 lateral view by the broad preopercular. In contrast with the Osteoglossidae, the nasal 

 bones are very slender and laterally placed, the prominent mesethmoid being in contact 

 with the frontals. There are no posttemporal fenestrae, no supramaxilla, no angular bone 

 in the mandible. A peculiar feature is that the ectosteal articular remains suturally distinct 

 from the endosteal or true articular (Ridewood, I904f, p. 208). 



At first sight the skull of Hyodon suggests relationships with that of Osteoglossum. In 

 both the circumorbital series is reduced in front of the orbit and much enlarged behind it. 

 Both lack the supramaxillae and have downwardly projecting teeth on the parasphenoid. 

 In both the interopercular is concealed from the outer side by the preopercular. But 

 along with these and other resemblances there are many differences: the marked reduction 

 of the nasals in Hyodon, the development of prominent pegs on the parasphenoid for 

 articulation with the entopterygoid in Osteoglossum, and so forth. 



Notwithstanding these and other differences noted by Ridewood, the construction of 

 the pectoral girdle in Hyodon 'shows the most unmistakable marks of affinity with that of 

 Osteoglossum. 



Notopterus. — This genus affords a fine example of mutual adjustment of skull form 

 and body form. In Notopterus chiiala (Fig. 61) the concave dorsal contour of the skull 

 rises steeply toward the high hump on the back, the great size of which must aid in the 

 balance of the body when propelled by the undulations of the much elongated anal fin. 

 The rear of the skull has thus increased greatly in height. As the orbits have moved for- 

 ward to the front of the head, while the hyomandibular has as usual retained its articulation 

 at the back part of the lateral wall of the braincase, the middle part of the skull behind the 

 orbits has become much elongated, more so in this species than in Notopterus kapirat, as 

 figured by Ridewood. 



In top view the skull is long and narrow, surmounted in its posterior half by several 

 crests that mark the boundaries between forwardly-extended strips of body muscles. The 

 long supraoccipital is separated from the frontals by the short parietals. 



In the side view, primitive isospondyl features, such as the extension of the lateral 



