ing at least three gills, of the first to third arches); three 

 epibranchials (of the second to fourth arches); two 

 pharyngobranchials (at least of the second and third 

 arches; except in diodontids, of at least the first and sec- 

 ond arches); supracleithrum; cleithrum; postcleithrum 

 as at least a single piece; coracoid; scapula; three acti- 

 nosts (second to fourth sequentially from anterodorsal to 

 posteroventral); between 9 and 25 pectoral fin rays (fin 

 placed in about middle of side of body); between 7 and 52 

 dorsal fin rays and a similar or only slightly lesser num- 

 ber of basal pterygiophores; between 7 and 66 anal fin 

 rays and a similar or only slightly lesser number of basal 

 pterygiophores; between 16 and 30 vertebrae (usually 

 less than 24); gill rakers on the anterior and posterior 

 edges of the first to third arches. 



Those elements which are present in some plec- 

 tognaths, but not in others, are: basisphenoid (con- 

 sistently present only in triodontids and molids, but rare- 

 ly present in at least one species of triacanthodid); 

 sesamoid articular (absent only in several species of 

 tetraodontids and molids); basihyal (present only in 

 triacanthodids); dorsal hypohyal (absent only in a few 

 diodontids and many tetraodontids); interhyal (absent 

 only in diodontids and most tetraodontids); second and 

 sixth branchiostegal rays (one or both absent only in a 

 few species of monacanthids and aracanids; the sixth 

 absent in one species of molid), urohyal (absent in all 

 Tetraodontoidei except triodontids); first and fourth 

 pharyngobranchials (pharyngobranchial of first arch 

 absent in monacanthids, molids, and some ostraciids; 

 that of the fourth arch absent in balistids, monacan- 

 thids, ostraciids, tetraodontids, and diodontids) and 

 third pharyngobranchial (absent only in a few diodon- 

 tids); teeth on the pharyngobranchials (that of the 

 first arch with teeth in diodontids and some tetra- 

 odontids; that of the second with teeth in all but a few 

 aracanids; that of the third with teeth in all but some os- 

 traciids and diodontids; that of the fourth with teeth in 

 triacanthodids, triacanthids, aracanids, triodontids, and 

 molids); teeth on fifth ceratobranchial (toothed in 

 triacanthodids, triacanthids, balistids, and triodontids 

 and with minute teeth in a few diodontids); 

 basibranchial and epibranchial of the first arch (absent 

 only in one species of monacanthid); first actinost (ab- 

 sent only in molids); pterosphenoid (absent only in one 

 species of tetraodontid); prefrontal (absent only in two 

 species of tetraodontids and several species of diodon- 

 tids); posttemporal (absent in one species of monacan- 

 thid and in all Tetraodontoidei); mesopterygoid (ab- 

 sent in most species of triacanthids and in one species of 

 tetraodontid); interoperculum (absent only in one 

 species of molid); pelvis (absent in aracanids, ostraciids, 

 tetraodontids, diodontids, molids); well-developed pel- 

 vic fin spine (present only in triacanthodids, with the 

 possible exception of several Eocene species, and triacan- 

 thids; thoracic in position, placed under pectoral fin 

 base); well-developed pelvic fin rays (present only in one 

 Eocene species of triacanthodid, with four rays); poorly 

 developed pelvic fin rays immediately following the spine 

 (one or two present in many species of triacanthodids; 



one even less well-developed ray present in some species 

 of triacanthids); a rudimentary but complex composite 

 pelvic fin element at the posterior end of the pelvis most- 

 ly hidden from view by enlarged encasing scales (present 

 only in balistids and many monacanthids); pleural ribs 

 (present only in triodontids, one species of monacan- 

 thid, and probably in one of the Eocene species of both 

 the triacanthodids and the tetraodontids); epipleurals 

 (present in all triacanthodids, with the possible exception 

 of one of the Eocene species, and in all triacanthids, 

 balistids, monacanthids, and triodontids); a separate 

 epural (absent in aracanids, ostraciids, and molids and 

 in at least most diodontids); uroneurals (present as one 

 or two pairs in triacanthodids, triacanthids, and triodon- 

 tids, and, rarely, in at least one species of balistid); a 

 separate parhypural (autogenous in triacanthodids, 

 balistids, monacanthids, triodontids, and tetraodon- 

 tids); one or more separate hypurals (usually five, 

 sometimes only three or four, separate hypurals in 

 triacanthodids, four in triodontids, one in triacanthids, 

 balistids, and tetraodontids, and one in at least most 

 monacanthids); a separate haemal spine of the penul- 

 timate vertebra (autogenous in triacanthodids, balistids, 

 monacanthids, aracanids, triodontids, tetraodontids, 

 and a few species of ostraciids); a separate haemal spine 

 of the antipenultimate vertebra (autogenous only in trio- 

 dontids); procurrent caudal fin rays (present in triodon- 

 tids and in at least one species of Eocene triacanthodid); 

 principal caudal fin rays (apparently absent in at least 

 two of the three species of molids, but perhaps represent- 

 ed in the other species by the rays in the central nipple of 

 the pseudocaudal fin otherwise formed of posteriorly 

 migrated dorsal and anal fin rays; 12 principal rays in 

 triacanthodids, triacanthids, balistids, monacanthids, 

 and triodontids, 11 in aracanids (10 in one species) and 

 tetraodontids, 10 in ostraciids, and 9 or 10 in diodontids; 

 in all cases the uppermost ray and the lowermost ray be- 

 ing unbranched and the intervening rays branched, ex- 

 cept in tetraodontids in which the uppermost ray and the 

 two lowermost rays are unbranched, and in one species of 

 monacanthid in which several rays both above and below 

 are unbranched); dorsal fin spines and their basal pte- 

 rygiophores (six spines, the last four of which may be 

 rudimentary and mostly buried beneath the skin, the 

 spines borne on five basal pterygiophores in triacantho- 

 dids; usually six spines, rarely only four or five, the sixth 

 spine nearly always a buried rudiment, the spines borne 

 on four, rarely three or five, basal pterygiophores in 

 triacanthids; three spines borne on two basal pteryg- 

 iophores in balistids; two spines, rarely only one, borne 

 on one basal pterygiophore in monacanthids; two, pos- 

 sibly three, rudimentary spines borne on two basal pte- 

 rygiophores in triodontids; no spines in aracanids, os- 

 traciids, tetraodontids, diodontids, and molids); 

 supraneural elements (one, as a strut supporting the sec- 

 ond basal pterygiophore, in balistids; one in aracanids 

 and ostraciids; one in several molids and in most tetrao- 

 dontids; absent in all monacanthids and diodontids); os- 

 sified Baudelot's ligament (present only in aracanids and 

 ostraciids); trituration teeth or teeth internal to the ma- 



