prerosphenoid hyomandibular 



parasphenoid 

 frontal 



Figure 254.— Chonerhinos 



modestus: lateral view of head, 



31.0 mm SL, Borneo. 



Figure 255.— Xenopterug 

 naritua: lateral view of head, 

 143 mm SL, Bay of Bengal. 



feature a moderate skull width and snout region length, 

 with the frontals wide posteriorly but gradually and 

 evenly narrowed anteriorly, and the sphenotics not 

 prolonged anterodorsally to form more than the rear 

 margin of the orbit, these being conditions as found in 

 the ancestral triodontids. In this view, species such as S. 

 maculatus, trichocephalus, and pachygaster have 

 relatively generalized skulls in which the frontals are of 

 moderate width and taper gradually anteriorly, while, 

 going in one direction, greeleyi is perhaps very slightly 



specialized in at least this one respect by the slightly 

 abrupt narrowing of the frontals at the level of the 

 prefrontals, while testudineus is more specialized by the 

 lesser anterior tapering of the frontals and the more 

 abrupt narrowing at the level of the prefrontals. The 

 testudineus-Uke condition could be ancestral to that of 

 annulatus through an increased expansion of the ante- 

 rior region of the frontals and an anterodorsal expansion 

 of the sphenotics. 

 The monotypic Guentheridia in the eastern Pacific 



