apparatus. All four have the specialized condition of the 

 absence of the prefrontal. However, the absence of a pre- 

 frontal is not entirely confined to these four species, for 

 C. orbicularis lacks the prefrontal but has nine caudal 

 rays, a spine on the forehead, and a nasal tube with two 

 nostrils. In C. orbicularis the palatines are more elon- 

 and less widely spaced than in the other species of 

 Chilomycterus, and the frontals are deeply indented in 

 the region where prefrontals ordinarily occur. 



The minor variation in the caudal fin supporting struc- 

 tures in diodontids is discussed by Tyler (1970b), this in- 

 volving the degree of fusion between the epural and the 

 neural spine of the penultimate vertebra and between 

 the parhypural variously to the fused hypural-centrum 

 plate or to the haemal spine of the penultimate verte- 

 bra. These regions are so difficult to interpret in the spec- 

 imens examined that no information of diagnostic or 

 phylogenetic usefulness was found. 



In many diodontids, as in all other gymnodonts except 

 Triodon, the fifth ceratobranchial is toothless. However, 

 in C. affinis, orbicularis, and tigrinus the fifth cerato- 

 branchial bears a small patch of minute teeth. In the sin- 

 gle large specimen of C. reticulatus examined a small 

 patch of minute teeth is present on the fifth ceratobran- 

 chial on one side but not on the other. The two species of 

 Dicotylichthys examined (nicthemerus and punctula- 

 tus) both have a well-developed elongate patch of minute 

 teeth on the fifth ceratobranchials. These small teeth 

 could either be remnants of those that probably occurred 

 on the fifth ceratobranchial of the ancestral tetraodon- 

 tid line, retained from its triodontid ancestry before the 

 fourth gill was lost, or a de novo acquisition in a few dio- 

 dontids from an immediate ancestry lacking teeth on the 

 fifth ceratobranchial. The latter seems more reasonable, 

 for the loss of teeth on the fifth ceratobranchial is un- 

 doubtedly associated with the loss of the fourth gill and 

 of the gill slit between the fourth and fifth arches, which 

 event occurred in the ancestral line common to both the 

 tetraodontids and diodontids, and both families prob- 

 ably have a long history of lacking teeth on the fifth cer- 

 atobranchial while still retaining the ability to occasion- 

 ally develop teeth there as dietary changes took place in 

 various evolutionary lines making use of slightly differ- 

 ing foods. 



