epiotics; 9) the development of a posterior prolongation 

 of the pterotics broadly articulating with the supra- 

 cleithrum; 10) the dorsal prolongation of the basioc- 

 cipital excluding the exoccipitals from contact with the 

 first vertebra; 11) the loss of haemal arches and spines on 

 at least some of the abdominal vertebrae and the reduc- 

 tion in size of their neural spines; 12) the loss of all of the 

 supraneural elements and the vast rearrangements of the 

 vertebral column and basal pterygiophores of the dorsal 

 and anal fins in the formation of the continuous fin 

 around the posterior end of the body with the abortion of 

 the true caudal fin and its supporting structures and the 

 development of a pseudocaudal fin of posteriorly 

 migrated dorsal and anal fin rays and their basal ptery- 

 giophores; 13) reduction in the number of vertebrae from 

 20 to between 16 and 18; 14) reduction in the size of the 

 olfactory apparatus; 15) development of thickened or 

 hardened skin; 16) reduction in the size of the gill open- 

 ing; and 17) the loss of the air bladder. 



The tetraodontid-diodontid line of radiation from the 

 pre-Recent Triodon level of organization diversified 

 greatly, with well over a hundred species alive today, and 

 have become in many ways just as morphologically dif- 

 ferent if not more so from triodontids as have molids, 

 while retaining none of the few generalized characters 

 shared by triodontids and molids, even though in general 

 body form the tetraodontid-diodontid line is more 

 similar to that of Triodon (exclusive of the expansible 

 dewlap) than is that of molids. 



The major ways in which tetraodontids and diodon- 

 tids differ from Triodon, other than those previously 

 mentioned that distinguish Triodon from all other gym- 

 nodonts, whether they be the generalized triacanthodid- 

 like features of Triodon that are lost or the specialized 

 features of Triodon developed after the radiation of the 

 other gymnodonts from a triodontid ancestral group, 

 are: 1) the loss of the basisphenoid; 2) the loss of the 

 fourth gill and of the gill slit between the fourth and fifth 

 arches; 3) the loss of at least well-developed teeth on the 

 fifth ceratobranchial and on one of the three pharyngo- 

 branchials; 4) the loss of the pharyngobranchial of the 

 fourth arch; 5) the loss of gill rakers behind the anterior 

 edge of the fourth arch; 6) the great expansion of the first 

 branchiostegal ray into a horizontal pumping plate ar- 

 ticulated to the medial surface of the ceratohyal; 7) the 

 development of an inflatable diverticulum of the gut; 8) 

 the suturing of the ceratohyal with the epihyal; 9) the 

 frequent loss of the interhyal and dorsal hypohyal; 10) 

 the loss of at least most of the dorsal roof of the myo- 

 dome and of the posterior opening into it; 11) the loss of 

 all, or all but one, of the supraneural elements; 12) the 

 development in one of the two families of long rodlike 

 dental units and of the frequent reduction in size or loss 

 of the trituration plates or teeth; 13) the development of 

 a more limited area of articulation of the palatine with 

 the cranium either at a notch and flange between the 

 palatine and ethmoid-vomerine region or by extensive 

 suturing between the palatine and frontal; 14) a reduc- 

 tion in the massiveness of the prefrontal and supra- 

 cleithrum; 15) the usual better development of the 



mesopterygcid (except absent in one species) and its firm 

 suturing to the metapterygoid as well as to the palatine; 

 and 16) the development in three or more of the ab- 

 dominal vertebrae of bifid neural spines, always in- 

 cluding the first three abdominal vertebrae. 



Thus, even though Triodon is by far the most 

 generalized gymnodont and retains many primitive 

 features of its triacanthodid ancestry, the structure of the 

 only species of which we have knowledge other than jaw 

 fragments, the Recent Triodon macropterus, is too 

 specialized for an Eocene fish closely similar to it to have 

 been the ancestral line from which the molid and tetrao- 

 dontid-diodontid lines radiated. This ancestral triodon- 

 tid line had a more generalized organization than that of 

 T. macropterus, not yet possessing such specializations 

 as the small size and entirely orbital placement of the 

 sphenotics allowing the frontals and exoccipitals to meet, 

 and the elongation and tapering of the caudal peduncle 

 and associated vertebral modifications. Nevertheless, an 

 early Eocene Triodon-like fish minus the specializations 

 as seen in the single Recent species is undoubtedly an- 

 cestral to the other two major subgroups of gymnodonts 

 and itself evolved from the eoplectin triacanthodids in 

 the early Eocene. 



Infraorder Tetraodontoideo 



Comparative diagnosis (contrast with that of the 

 Triodontoideo).— Spiny dorsal fin completely absent 

 and no more than one supraneural element present; ribs 

 and epipleurals absent; caudal fin with 11 or fewer prin- 

 cipal rays and no procurrent rays; caudal fin varying 

 from moderately forked to rounded; caudal skeleton with 

 no more than one separate hypural, and no uroneurals 

 and hypurapophysis; haemal spine of antipenultimate 

 vertebra fused to its centrum; neural and haemal spines 

 of penultimate vertebra various but not long rounded 

 shafts directly supporting caudal fin rays; caudal pedun- 

 cle not distinctly tapered to narrow transversely in- 

 dented regions above and below just in front of the 

 caudal fin, the least depth of the peduncle being about 

 6% SL or greater, and the least width at this region al- 

 ways less than the least depth; none of the caudal verte- 

 brae with anterolateral processes above the centra; no 

 pelvis; no expansible dewlap of skin in front of the anus, 

 but inflatability of the abdominal region present in all 

 families except molids; cleithrum not elongate an 

 teriorly, reaching forward no further than about the level 

 of the middle of the ceratohyal; basisphenoid either ab 

 sent or present as a large plate in the interorbital sep 

 tum; myodome either absent or present only as a rudi 

 ment of the dorsal roof represented by medial wings ol 

 the prootics, the roof always highly incomplete; no chan 

 nel present between the apposed surfaces of the para 

 sphenoid and basioccipital; scapular foramen incom- 

 plete, closed anteriorly by the cleithrum; scapula 

 without a distinct knob for articulation with the upper- 

 most pectoral fin ray; three or four actinosts, if four pre- 

 sent at least some sutured either to one another or to the 



