The Eoplectinae differ from the other fossil subfamily 

 of triacanthodids, the Spinacanthinae, mainly by having 

 a larger eye located about midway between the snout and 

 spiny dorsal fin origin, the lesser development of the 

 spiny dorsal fin and its shorter base and more posterior 

 position, better developed and longer based soft dorsal 

 and anal fins, and the massive beaklike jaws. 



The anatomical diversity of the two Recent sub- 

 families of triacanthodids is summarized below from 

 Tyler (1968). Of greatest phylogenetic significance is the 

 arrangement and form of the bones of the posterodorsai 

 region of the skull and in the shape of the pelvis, which 

 features divide the Recent species into different sub- 

 families. 



In the Hollardiinae, which form a compact group of 

 five species in two genera, the supraoccipital is domelike 

 and separates the epiotics on the dorsal surface of the 

 skull, while the epiotics directly contact the frontals and 

 the pelvis is a sturdy shaft with a more or less triangular 

 shape in cross-section. In the Triacanthodinae, whose 14 

 species in 9 genera range from relatively normal species 

 through intermediates to weirdly specialized forms with 

 extremely long tubular snouts, the supraoccipital 

 basically is a broad flat bone with a variously developed 

 crest and does not separate the epiotics on the dorsal sur- 

 face of the skull, while the epiotics are separated from 

 the frontals by the sphenotics and the pelvis is a broad 

 flat basin with upturned edges. 



The two genera of Hollardiinae differ mainly in that 

 Parahollardia retains a small number of teeth internal to 

 the major outer series while the inner teeth on the 

 premaxillary and dentary are lost in Hollardia. 



Similarly, in the Triacanthodinae, one genus 

 (Triacanthodes) retains inner series teeth which are lost 

 in the other eight genera. In the Triacanthodinae the 

 teeth are usually conical, as they are also in the Hollar- 

 diinae, but in a few genera the teeth are large, thin, and 

 wide, much compressed from front to back, and either 

 truncate (Tydemania) or truncate to rounded or pointed 

 (Macrorhamphosodes) distally. In Johnsonina the con- 

 ical teeth are smaller and far more numerous than in the 

 other genera of triacanthodins, while in Halimochi- 

 rurgus the teeth are small and conical but of low to 

 moderate (3 to 15) number in both jaws. In one species, 

 Macrorhamphosodes platycheilus, the few upper jaw 

 teeth are lost in adults. 



The snout in most triacanthodins is of short to 

 moderate length and normal shape, but in one of the two 

 species of Bathyphylax, the snout, although of only 

 moderate length, is decidedly concave in front of the 

 eyes, giving it a tubular appearance, while in Macro- 

 rhamphosodes and Halimochirurgus the snout is enor- 

 mously (26 to 49% SL; least so in young) elongate and 

 tubular. The mouth is more or less terminal in Triacan- 

 thodes, Mephisto, and Paratriacanthodes, as it is in the 

 Hollardiinae, but it is slightly supraterminal in John- 

 sonina and Atrophacanthus, slightly to decidedly supra- 

 terminal in Bathyphylax, and decidedly supraterminal 

 in Tydemania, Macrorhamphosodes, and Halimo- 

 chirurgus. 



The bones of the long tubular snouts of Macrorham- 

 phosodes and Halimochirurgus are strangely rearranged 

 from the condition of other members of the family. In 

 both species of Halimochirurgus and in M. uradoi the up- 

 per three-fourths of the tube is composed, from anteriorly 

 to posteriorly, more or less successively by the ecto- 

 pterygoids, palatines, and vomer, which are thin, curv- 

 ed, and elongate plates. In M. platycheilus, however, the 

 vomer extends further forward than in the others to form 

 the roof of the anterior region of the snout to the exclu- 

 sion of the ectopterygoids and palatines. The ventro- 

 lateral region of the tube in all four species is formed by 

 the elongate quadrate and the long slender anterior 

 prolongation of the preoperculum. The premaxillary 

 pedicels of M. uradoi are prolonged posteriorly as thin 

 filaments but in none of the four long-snouted species do 

 the pedicels extend posteriorly to the region of the 

 ethmoid, as they do in other triacanthodids. The bones of 

 the snout of Halimochirurgus more completely roof over 

 the tube than in Macrorhamphosodes, and the tube is 

 narrower and more elongate in Halimochirurgus than in 

 Macrorhamphosodes. 



In Halimochirurgus the lips are of normal 

 triacanthodid size and shape, while in Macrorham- 

 phosodes the lips form a wide disklike structure around 

 the mouth. The size of the lips in Tydemania is more or 

 less intermediate between that of Macrorhamphosodes 

 and other triacanthodids. In Macrorhamphosodes the 

 mouth becomes twisted to the left or right, usually 

 progressively so with increasing specimen size, while in 

 Halimochirurgus it is symmetrically placed at all sizes. 



The spiny dorsal fin is relatively well developed, with 

 all six spines protruding through the surface and of 

 gradually decreasing length posteriorly in the series in 

 Triacanthodes, Mephisto, Paratriacanthodes, and John- 

 sonina, as in the Hollardiinae, while in Bathyphylax the 

 fourth spine is much shorter than the third but protrudes 

 through the surface and is clearly seen, with the fifth and 

 sixth spines only slightly shorter than the fourth and 

 protruding only slightly less. In Atrophacanthus, 

 Tydemania, Macrorhamphosodes, and Halimo- 

 chirurgus the last three dorsal spines are rudimentary 

 and usually only barely, if at all, protrude through the 

 surface, while in Halimochirurgus the third spine also 

 tends to be rudimentary. 



Triacanthodes, Johnsonina, and Paratriacanthodes 

 herrei usually have two pelvic fin rays, although in large 

 specimens the second may tend to become a buried rudi- 

 ment and be resorbed, while in P. retrospinis, Mephisto, 

 Atrophacanthus, Bathyphylax, Tydemania, Macro- 

 rhamphosodes, and Halimochirurgus there is only a single 

 pelvic fin ray. 



The size of the gill opening varies from relatively huge 

 for a plectognath fish in Mephisto, in which the slit ex- 

 tends down well below the level of the lower edge of the 

 pectoral fin base, to simply large, as in Triacanthodes, 

 Halimochirurgus centriscoides, and Macrorham- 

 phosodes platycheilus, in which the slit extends down to 

 or around the level of the lower edge of the pectoral fin 

 base, to moderate or short, as in all the other species, in 



