the plectognaths only in the suborder Gymnodontes 

 (Tetraodontoidei), and it is closely similar to that of 

 Triodon, the monotypic representative of the Triodon- 

 tidae and the most generalized member of its suborder. 



Eoplectus is similar to Recent triacanthodids and 

 differs from Triodon in the following ways: 1) Recent 

 triacanthodids have the most fully developed pelvic fin 

 among plectognaths, in more generalized forms con- 

 sisting of a spine and two unbranched rays, while in 

 Eoplectus this fin is even better developed as a spine and 

 four branched rays, the better development in Eoplectus 

 being less reduced than in triacanthodids from the 

 ancestral perciform condition of one spine and five 

 branched rays; 2) the rounded i, 10, i caudal fin, without 

 procurrent fin rays; 3) the short caudal peduncle; 4) six 

 dorsal fin spines borne on five well-developed basal 

 pterygiophores; 5) first basal pterygiophore of the spiny 

 dorsal fin articulated low on the base of the skull and 

 bearing the first two spines; 6) third to fifth basal 

 pterygiophores of the spiny dorsal fin with their shafts 

 directed ventrally or posteroventrally in relation to the 

 vertebral axis and articulated with the neural spines of 

 the fifth to eighth vertebrae; 7) 20 vertebrae; 8) probably 

 no ribs; 9) small prefrontal not extensively sutured to the 

 frontal, parasphenoid and ethmoid; 10) moderately large 

 eye located about midway between the snout and spiny 

 dorsal fin origin; 11) relatively large numbers of dorsal 

 and anal fin rays. 



Eoplectus is similar to Triodon and differs from the 

 other triacanthodids in the following ways: 1) the small 

 individual rounded dental units intimately associated 

 with the matrix of the premaxillaries and dentaries to 

 form a crushing beak; 2) premaxillary probably im- 

 movably held by suturing to the maxillary, the ascending 

 process of the premaxillary poorly developed and the up- 

 per jaw probably nonprotrusible, rotating around a ball 

 and socket joint between the palatine and the maxillary; 

 3) the palatine probably immovably sutured to the 

 ethmoid-vomerine region; 4) vertebrae in a 9 -I- 11 ar- 

 rangement. 



Eoplectus is similar to both the other triacanthodids 

 and to Triodon in the following ways: 1) the pelvis with 

 a long anterodorsal shaft probably held between the 

 anteromedial edges of the cleithra; 2) a generalized 

 caudal skeleton with the parhypural, numerous 

 hypurals, and single epural probably not fused to one 

 another or to the last centrum. 



Eoplectus differs from both the other triacanthodids 

 and Triodon in the following ways: 1) probably no 

 uroneurals, or only very small ones at the most; 2) the 

 great depth of the caudal peduncle; 3) the placement of 

 the well-developed shaft of the second basal pterygio- 

 phore of the spiny dorsal fin, directed strongly antero- 

 ventrally and articulating either with the base of the 

 skull or between the neural spines of the first and second 

 vertebrae; 4) the spiny dorsal fin base equal in length to 

 the soft dorsal fin base rather than either much greater or 

 much less than it; 5) soft dorsal and anal fin rays far 

 more numerous than the basal pterygiophores supporting 

 them; 6) the extremely well developed ventral flange of 



Figure iS.—Zignoichthys oblongus: 



lateral view of holotype, ca. 161 mm SL, 



Eocene of Monte Bolca, Italy. 



the parasphenoid versus weakly to moderately 

 developed; 7) the highly arched vertebral column; 8) the 

 body with an incomplete covering of scales, many of 

 which are isolated and nonoverlapping. 



The other species of Eoplectinae is the extremely poor- 

 ly known Zignoichthys oblongus (Zigno 1874a, b) from 

 the same strata of the Eocene of Monte Bolca, Italy, in 

 which Eoplectus occurs. Of its few known features, re- 

 described by Tyler (1973b) and briefly summarized here, 

 the most important is the huge jaw bone composed of 

 two equal sides fully fused together in the midline with 

 no evidence of any kind of separation or suture between 

 the right and left halves. The biting edge of the beaklike 

 jaw is not exposed, but the teeth can be expected to be 

 small dental units incorporated into the matrix of the 

 bone much as in Eoplectus, Triodon, and some of the 

 other gymnodonts. The body is covered with numerous 

 small scales with numerous upright spinules, and the 

 caudal fin has about 12 principal rays in addition to at 

 least two or three procurrent rays both above and below. 

 Among plectognaths, procurrent rays are otherwise 

 known only in Triodon. It is not known with surety 

 whether dorsal fin spines were present, for most of the 

 appropriate region of the body is missing, but it is 

 probably best to assume that a spiny dorsal fin of un- 

 known size was present. The region where one would ex- 

 pect to find a pelvic fin if one were present is relatively 

 well preserved, and no pelvic fin is apparent, and it can 

 probably be assumed that the pelvic spine, if present at 

 all, was of reduced size. 



Too few critical features are known of Zignoichthys to 

 allow a precise placement of it among the plectognaths, 

 and it has very tentatively been associated (Tyler 1973b) 

 with Eoplectus and the Eoplectinae of the Triacanthodi- 

 dae for the following reasons. It has a generalized condi- 

 tion of the caudal fin and caudal peduncle, and of the 

 soft dorsal and anal fins and scales consistant with the 

 triacanthodids, and, if a spiny dorsal fin was present, it 

 would probably have been longer based than the soft dor- 

 sal fin, judging from the space available for it, while the 

 jaw that is exposed is similar to that of Eoplectus. 



