the last vertebral centrum. The uroneural is located 

 between the epural and free hypural and has a tube 

 through its length enclosing the neural canal. The tube 

 also extends through the distal region of the epural to 

 exit near to the tip of the neural spine of the penul- 

 timate vertebra. A large foramen just below the middle of 

 the fused hypural plate represents the region of fusion of 

 the parhypural with the hypurals above it, but other- 

 wise the hypurals (except for the uppermost free ele- 

 ment) and the parhypural are indistinguishably fused 

 with the last vertebral centrum. As explained by Tyler 

 {1970b: 10) the single uroneural probably represents the 

 second pair of uroneurals of triacanthodids, in which the 

 first of the two pairs usually is much smaller and more 

 variable in occurrence than is the second pair. A promi- 

 nent horizontal crest is present on the fused centrum- 

 parhypural-hypural plate for muscle attachment. 



Caudal fin rays. —Twelve in number; the upper- 

 most ray and the lowermost ray unbranched, the others 

 becoming increasingly branched toward the two middle 

 rays, which are branched in triple to quadruple 

 dichotomies. The upper unbranched ray has its bifur- 

 cate base articulated with the distal end of the free 

 hypural and the extreme dorsal end of the fused hypural 

 plate. The uppermost two or three branched rays have 

 their bifurcate bases overlapping the fused hypural 

 plate, whereas the middle four to five rays to not overlap 

 the plate. Similarly, the lower unbranched ray slightly 

 overlaps the fused parhypural-hypural plate, and the 

 branched rays above increasingly less so. 



DORSAL AND ANAL FINS. 



Dorsal Fin. 



Spines and pterygiophores. — Six spines of greatly 

 decreasing length posteriorly in the series and only four 

 basal pterygiophores, without the intervention of os- 

 sified distal pterygial elements. The first two spines are 

 borne on the first basal pterygiophore, whereas the third, 

 fourth, and fifth spines are borne on their own indivi- 

 dual basal pterygiophores. There is no basal pterygio- 

 phore supporting the minute rudimentary sixth spine 

 which is buried beneath the skin. The concave medial 

 region of the base of the first spine rotates over a low 

 medial flange at the dorsal surface of the first basal pte- 

 rygiophore. However, this medial flange more posteriorly 

 becomes a tall, somewhat anteriorly directed prong 

 which fits into a transverse hole through the base of the 

 spine. It is around this dorsal flange of the pterygiophore 

 that the first spine rotates. The spine can be erected and 

 locked in any of a continuous series of positions by the 

 close apposition of the base of the first spine with the dor- 

 sal articular surface of the first pterygiophore on either 

 side of its medial flange, both surfaces being somewhat 

 rough and irregular. This mechanism is basically the 

 same as that described for the triacanthodid Parahol- 

 lardia lineata, although the medial flange of the basal 



pterygiophore in the case of triacanthids is much less 

 elaborately developed than it is in triacanthodids (see 

 Tyler 1968:fig. 7 for a comparative illustration of the 

 structure in the two families of triacanthoids). The sec- 

 ond spine articulates basally with the dorsal surface of 

 the first basal pterygiophore just behind the posterior 

 edge of the anterodorsal flange around which the first 

 spine rotates, at a slight concavity on the surface of the 

 pterygiophore in this region. This slight concavity has a 

 very small medial flange fitting into the concave base of 

 the second spine. The third to the fifth spines articulate 

 at their generally rounded basal ends with simple shal- 

 low concavities in the dorsal surfaces of their individual 

 pterygiophores. The sixth spine is rudimentary and has 

 no basal pterygiophore supporting it. 



The first basal pterygiophore is a very stout shaft of 

 bone with only narrow anteromedial and posteromedial 

 flanges for muscle attachment. Basally the stout shaft is 

 firmly held in place by fibrous tissue within the deep 

 bony well formed posteriorly by the neural spine of the 

 first abdominal vertebra and anteriorly by the exoc- 

 cipitals. This is a relatively immovable articulation. The 

 second to fourth basal pterygiophores are much smaller 

 than is the stout first pterygiophore and each bears its 

 own individual dorsal spine of decreasing length in the 

 series. The four basal pterygiophores articulate to one 

 another by fibrous tissue or, in some cases, by a very 

 slight interdigitation of their apposed surfaces. The sec- 

 ond to fourth pterygial elements are cartilage filled at 

 their extreme ventral ends, at the end of the vertical 

 shaft along their lateral surfaces. The second and third 

 pterygiophores also often remain cartilaginous at the ex- 

 treme posterodorsal tip. 



Fin rays and pterygiophores. — Usually 21 or 22 fin 

 rays are present; the first 1 or 2 rays unbranched, the 

 others branched in single to double dichotomies. No os- 

 sified distal pterygiophores are present between the 

 bifurcate bases of the rays in the specimens of P. 

 strigilifer studied, but in some other species of 

 triacanthids ossified distal pterygiophores are present, at 

 least in large adult specimens (see Tyler 1968:268). When 

 ossified distal pterygiophores are present they are a 

 single piece in the midline rather than a pair of pieces as 

 found in triacanthodids. Basally the fin rays are sup- 

 ported by a number of basal pterygiophores similar in 

 number to that of the fin rays. The pterygiophores ex- 

 tensively interdigitate with one another in their upper 

 regions just below the apposed concavities in their 

 anterior and posterior edges which give rise to the round 

 holes between the distal heads of the pterygiophores that 

 are present at least anteriorly in the series. Lateral 

 flanges for muscle attachment are very well developed 

 along the lengths of the pterygiophores. These flanges are 

 widest toward the distal end of the pterygiophore where 

 they form prominent hooklike processes. The first basal 

 pterygiophore of the soft dorsal fin usually is located 

 between the neural spines of the fourth and fifth ab- 

 dominal vertebrae, while the last few basal pterygio- 

 phores lie between the neural spines of the fifth and sixth 



