Figure 252.— Chelonodon 



fluviatilia: lateral view of head, 



84.7 mm SL. Thailand. 



metapterygoid 

 mesopterygoid 



Between these two extremes of dorsalis and annulatus, 

 however, are other species of Sphoeroides which form an 

 almost continuous series of intermediate conditions. 

 Thus, S. lobatus, angusticeps, and nephelus have only 

 slightly wider skulls and slightly less elongate snout 

 regions than S. dorsalis and otherwise are similar to it, 

 while a number of other species (S. spengleri, maculatus, 

 and greeleyi) have progressively wider skulls and less 

 elongate snout regions, leading to species such as S. 

 trichocephalus and pachygaster with wider frontals, 

 progressively less slender or tapering anteriorly, and 

 eventually to S. testudineus in which the frontals are 

 only slightly tapered anteriorly above the orbit, not being 

 greatly narrower there than posteriorly, and only taper- 

 ing to a point far anteriorly and abruptly at the rear edge 



of the prefrontals. The above series of progressively wider 

 skulls does not necessarily indicate precisely similar 

 specific relationships. 



Among the species of Sphoeroides the condition of the 

 skull in testudineus is that which most closely ap- 

 proaches that of annulatus. In fact, these two species are 

 usually considered to be closely related geminates to 

 each side of the Central American isthmus {annulatus in 

 the eastern Pacific and testudineus in the Gulf of Mexico 

 and the Caribbean) which have retained relatively simi- 

 lar color patterns of pale circles and reticulations unique 

 in the genus. 



One would suspect that the extremes of dorsalis and 

 annulatus both represent specializations, and that a 

 generalized skull structure in tetraodontids would 



