in the Monacanthidae and in the triacanthodid sub- 

 family Triacanthodinae. In the monacanthids the highly 

 specialized genus Psilocephalus could be considered as 

 subfamilially distinct (as it sometimes has been in the 

 past) from the monacanthids, and is not so recognized 

 here mainly because some of the intermediates between 

 its structure and that of more generalized monacanthids 

 are found in some of the Alutera-Vike fishes. Similarly, 

 the highly specialized monacanthid Pseudaluteres could 

 be considered subfamilially distinct, and is not so recog- 

 nized here mainly because it is clear that Oxymona- 

 canthus is intermediate between it and the more 

 generalized monacanthids, and an Oxymonacanthus-\ike 

 form gave rise to it. In the triacanthodids, the two long- 

 snouted highly specialized genera of the Recent sub- 

 family Triacanthodinae could each be recognized as a 

 distinct subgroup (tribe) and are not so recognized here 

 mainly because a different genus is intermediate 

 between each of the two long-snouted genera and the 

 more generalized triacanthodins, i.e., a Bathyphylax-like 

 line is ancestral to Halimochirurgus and one like Tyde- 

 mania to Macrorhamphosodes. 



Relationships to the Diodontidae and to the Other 

 Tetraodontoidei. — The ways by which a triodontidlike 

 ancestral group probably gave rise to two major lines of 

 radiation, one leading to the tetraodontids and dio- 

 dontids and the other to the molids, are discussed under 

 the Triodontidae. The molids undoubtedly branched off 

 the common ancestral line at a more generalized level of 

 triodontidlike organization than did the tetraodontoids, 

 for molids retain a significant number of important 

 generalized features found in triodontids but not in tetra- 

 odontoids, even though molids superficially seem far 

 removed from triodontids and almost totally specialized. 

 In fact, the tetraodontoids are at least as highly specializ- 

 ed and as anatomically far removed from the ancestral 

 triodontidlike fishes as are the molids, as further discuss- 

 ed under the Triodontidae. 



In the vast majority of ways that tetraodontids differ 

 from diodontids, tetraodontids retain more generalized, 

 triodontidlike, conditions than do diodontids. But there 

 are important exceptions in which diodontids have the 

 more generalized conditions, indicating that the division 

 of the ancestral line into tetraodontids and diodontids 

 took place early in the evolution of the superfamily 

 (Eocene, on the basis of the fossil record), with diodon- 

 tids retaining a few generalized features while overall 

 becoming far more specialized than the tetraodontids. 



As previously discussed, it seems reasonable to assume 

 that the ancestral eoplectins that gave rise to the trio- 

 dontids and other gymnodont lines did so at a time when 

 the premaxillaries and dentaries were not yet fused to 

 their opposite members, but only interdigitated to them, 

 and with the teeth in the biting edge of the jaws small 

 rounded units incorporated into the matrix of the bone 

 but retaining much of their individual identity. It is here 

 assumed that the line of triodontidlike fishes which gave 

 rise to the tetraodontoids had the premaxillaries and 

 dentaries unfused and that the fusion of these bones to 



their opposite members in diodontids and molids has 

 been independent. It is possible that the triodontidlike 

 fishes which gave rise to the molids already had the den- 

 taries fused, as in Triodon and at least Zignoichthys of 

 the eoplectins, and that molids became further specializ- 

 ed by having the premaxillaries fused as well (along with 

 numerous other specializations, including the loss of dis- 

 tinct teeth in the biting edge of the jaws). 



While diodontids have the more specialized condition 

 of fused premaxillaries and dentaries in relation to tetra- 

 odontids having both of these bones articulated to their 

 opposite members by interlocking emarginations, the fu- 

 sion of these two jaw bones is a relatively uncomplex 

 event in comparison to the changes in the dental units in- 

 corporated into the matrix of the bone. Diodontids retain 

 the generalized condition of small rounded dental units 

 in the biting edge, just as in triodontids and eoplectins, 

 while tetraodontids have extremely specialized teeth, 

 perhaps the most specialized among teleosts, these being 

 long, slender rods lying obliquely transverse to the body 

 axis, parallel to the biting edge and incorporated into the 

 matrix. The large trituration plates present in the upper 

 and lower jaws of diodontids are of about the same size as 

 those of Triodon, even though there are usually fewer 

 series of teeth involved in the formation of the composite 

 plates (see discussion under the diodontid Chilomyc- 

 terus orbicularis). However, the form of the trituration 

 teeth and plates has probably been highly variable in 

 most groups of gymnodonts, correlated with changes of 

 diet in newly occupied habitats, and not too much im- 

 portance can be attached to the at least superficial 

 similarity of the trituration plates in Triodon and 

 diodontids, i.e., the trituration plates of diodontids are 

 not necessarily to be considered generalized. 



Other than the form of the teeth of the biting edge of 

 the jaws, diodontids are more generalized and Triodon- 

 like than tetraodontids only by: 1) the more frequent 

 presence of both a dorsal and ventral hypohyal; 2) the 

 presence of a prominent anteriorly directed prong on the 

 suboperculum attached by ligament to the interoper- 

 culum; 3) having none of the basal pterygiophores of the 

 dorsal and anal fin interdigitated to one another distally; 

 and 4) the presence of a prominent lateral flange on the 

 fused hypural plate, if this is homologous with the 

 hypurapophysis of Triodon, which is debatable. 



In all of the other far more numerous characters listed 

 here in the comparative diagnoses of the tetraodontids 

 and diodontids, the conditions found in the tetra- 

 odontids are the more generalized and the least far 

 removed from those in the ancestral triodontids (pre- 

 Recent Triodon level of organization). 



The diodontids must thus be considered an early 

 Eocene offshoot of the basal tetraodontids, with most of 

 the diodontid specializations centering around a more 

 highly defensively armoured exoskeleton variously of 

 long erectile quills or short nonerectile spines with 

 massive bases forming an open quilted carapace around 

 a relatively slow swimming body with a reduced caudal 

 region and usually relatively more massive jaws with a 

 greater crushing and grinding function. 



