Family Triacanthidae 



Comparative dia^osis (contrast with that of the 

 Triacanthodidae) (modified from Tyler 1968:234- 

 236). — A compact, strongly sutured skull and a mus- 

 cular body built for active swimming near the bottom; 

 very little cartilage visible on the external surface of the 

 skull between the regions of apposition of the bones in 

 the otic and occipital regions, the bones usually strongly 

 sutured to one another along all edges of proximity; an 

 outer series of about 8 to 10 heavy incisor teeth in each 

 jaw, internal to which are several more or less molariform 

 teeth, usually four (two in Trixiphichthys weberi) in 



Figure 31.— Typical body form in the Recent 

 Triacanthidae: Paeudotriacanthus strigilifer. 



the upper jaw and two in the lower; a large sturdy eth- 

 moid-frontal complex for the support of the massive den- 

 tition; the prefrontal with a long anterior extension, 

 sutured medially to the ethmoid and anteriorly to the 

 posterolateral extensions of the vomer; premaxillary 

 pedicels when fully retracted reaching only to the an- 

 terior basal region of the ethmoid, well-separated from 

 the frontals (probably least so in the Eocene 

 Protacanthodes); supraoccipital domelike, its posterior 

 surface concave; epiotics separated on the dorsal surface 

 of the skull by the supraoccipital and meeting medially 

 only on the posterior surface of the skull and ar- 

 ticulating anterolaterally with the frontals; ptero- 

 sphenoids meeting and suturing in the midline of the 

 posterior wall of the orbit; neural processes of the first 

 vertebra meeting and suturing in the midline above the 

 neural canal, forming with the exoccipitals a completely 

 enclosed bony well (with a bony bottom) in which the 

 shaftlike end of the first basal pterygiophore of the spiny 

 dorsal fin is immovably held (a different arrangement 

 present in the Oligocene Cryptobalistes); parasphenoid 

 in region of orbit with a dorsal arch and a well-developed 

 ventral flange below the orbit, the flange about 2 or 3 

 times as deep as the upper shaftlike portion of the bone 

 (except in the Eocene Protacanthodes, the shaft relative- 

 ly straight and the flange not much deeper than the 

 shaft); hyomandibular with a well-developed groove and 

 crest along its lateral surface more or less transversely; 

 pterotic with a ventral process overlying and firmly ar- 

 ticulating with the upper posterior portion of the 

 hyomandibular; supracleithrum placed vertically to the 



horizontal axis of the skull, nearly its entire length over- 

 lying the cleithrum; mesopterygoid absent (at least in 

 adults) in all but one species {Trixiphichthys weberi), 

 and when present small and difficult to distinguish from 

 the metapterygoid, to which it is mostly fused; olfactory 

 cavity between the ethmoid and prefrontal well defined, 

 with distinct bony boundaries; basihyal absent; lower 

 two branchiostegal rays enlarged (except in the Eocene 

 Protacanthodes), much wider than the upper branchios- 

 tegal rays; operculum elongate, usually widest in the 

 middle, not triangular; air bladder thick walled, 

 somewhat elongate, large and extending posteriorly al- 

 most the entire length of the abdominal cavity; pelvis a 

 sturdy shaft; the two halves of the pelvis variously fused 

 or extensively interdigitated, like a railroad rail in cross 

 section just behind the level of the pelvic spine (except in 

 the Oligocene Cryptobalistes, which has a basinlike pel- 

 vis); a single oblique crest on the side of the pelvis at the 

 level of the flange of the pelvic spine, allowing for two, 

 and only two, positions of locking the erected spine 

 (probably a different arrangement in the Oligocene 

 Cryptobalistes); the ventrolateral surface of the pelvis at 

 the base of the spine without a complete foramen and the 

 two sides of the base of the spine not meeting medially; 

 all haemal arches and spines of adults fully fused to their 

 centra; only the epural and uppermost hypural as 

 separate elements articulated by fibrous tissue to each 

 other and to the centrum, the other hypurals and the 

 parhypural fully fused to one another and to the cen- 

 trum (probably some separation of the middle hypurals 

 in the Eocene Protacanthodes); a single pair of 

 uroneurals present; thick epipleurals present from the 

 third or fourth abdominal vertebra to the first or second, 

 sometimes the third and rarely the fourth, caudal 

 vertebra; fifth basal pterygiophore of the spiny dorsal fin 

 usually absent, rarely present as a tiny buried bony 

 splint beneath the usually rudimentary sixth dorsal 

 spine; first basal pterygiophore of the spiny dorsal fin a 

 stout shaft (except very short in the Oligocene Crypto- 

 balistes) with poorly developed anterior and posterior 

 medial flanges, the flanges never wider than the shaft; 

 first basal pterygiophore of spiny dorsal fin with a medial 

 flange dorsally not completely enclosing a foramen; sec- 

 ond to fourth basal pterygiophores of spiny dorsal fin 

 reduced in size, their shafts not reaching ventrally to 

 between the tips of the neural spines (but associated with 

 those of the first to third vertebrae, except in the Eocene 

 Protacanthodes, in which the shafts reach to the distal 

 ends of the neural spines of the second to fourth 

 vertebrae) and the fourth pterygiophore occasionally 

 missing; none of the basal pterygiophores of the spiny 

 dorsal fin sutured to one another distally; spiny dorsal fin 

 base (including rudiments) much shorter than soft dor- 

 sal fin base (except the Eocene Protacanthodes, with 

 spiny dorsal base slightly longer than soft dorsal base); 

 only four or five (six in the Eocene Protacanthodes and 

 perhaps in the Oligocene Cryptobalistes) abdominal 

 vertebrae anterior to the first basal pterygiophore of the 

 soft dorsal fin; first anal fin basal pterygiophore with a 

 sturdy medial flange anterior to its lateral flanges, the 



