fibrous tissue with the epiotics and by slight interdigita- 

 tion with the exoccipitals. Over the anterior half of the 

 neural arch area just above its centrum, the first vertebra 

 articulates by fibrous tissue with the overlying posterior- 

 ly directed exoccipital condyles. The rim of the concave 

 anterior end of the centrum articulates with the rim of 

 the concave posterior end of the basioccipital. Posteriorly 

 the first vertebra articulates with the second vertebra by 

 apposition of the rims of their centra and by the short, 

 but wide, neural postzygapophysis of the first vertebra 

 slightly overlying the neural prezygapophyseal area of 

 the second vertebra. No haemal zygapophyses are pres- 

 ent. 



Other abdominal vertebrae. — In 17 specimens the 

 abdominal vertebrae numbered eight. Except for the 

 first vertebra, all of the abdominal vertebrae, as well as 

 the caudal vertebrae, have a bony roof over the neural 

 canal and a single, undivided, neural spine. The basal 

 regions of the neural spines and dorsal regions of the 

 neural arches become progressively more antero- 

 posteriorly expanded from the second to the eighth ab- 

 dominal vertebra. This expansion involves both the 

 neural pre- and postzygapophyses, which become 

 broadened into large articular surfaces, with the pre- 

 zygapophyses of the seventh and eighth (sometimes sixth 

 to eighth) abdominal vertebrae slightly overlying the 

 postzygapophyseal area of the vertebrae anterior to 

 them. No haemal pre- or postzygapophyses are present. 

 Each neural arch has a neural foramen along the middle 

 of its lateral surface. The first three abdominal vertebrae 

 have no transverse or haemal processes, but the fourth 

 and fifth vertebrae have transverse processes from the 

 anterolateral edges of their centra, the process of the fifth 

 being somewhat longer and more ventrally curved than 

 that of the fourth. The sixth vertebra has a slightly 

 longer transverse process, which differs from that of the 

 fourth and fifth vertebrae in that the two processes of the 

 sixth vertebra possess medial projections that meet and 

 are continuous in the midline beneath the centrum, 

 enclosing the haemal canal and thus representing a 

 haemal arch without a haemal spine. The seventh 

 vertebra has the haemal arch more strongly developed 

 and possesses ventrally directed processes from each side 

 of the haemal arch representing a bifid haemal spine. 

 The haemal apparatus of the eighth vertebra is like that 

 of the seventh, except larger. Six epipleurals or inter- 

 muscular bones are present in the myocommata between 

 the epaxial and hypaxial musculature, and borne 

 on the second to seventh abdominal vertebrae. The first 

 and second epipleurals articulate with the neural arches 

 of, respectively, the second and third abdominal 

 vertebrae. The third epipleural articulates with the dor- 

 sal surface of the short transverse process of the fourth 

 abdominal vertebra; the fourth epipleural with the dor- 

 sal surface of the transverse process of the fifth ab- 

 dominal vertebra. The fifth and sixth epipleurals articu- 

 late with the ventrolateral surfaces of the haemal arches 

 of, respectively, the sixth and seventh abdominal 

 vertebrae. 



Caudal Vertebrae. — In 17 specimens the caudal 

 vertebrae numbered 12. As is true with the last two or 

 three abdominal vertebrae, the first four to six caudal 

 vertebrae have the neural spine area around and above 

 the neural zygapophyses expanded into large articular 

 surfaces so that the neural prezygapophyses of the first 

 four or five caudal vertebrae overlie the neural postzyga- 

 pophyseal area of the vertebrae anterior to them. With 

 the exception of the last two vertebrae, each neural arch 

 has a completely enclosed neural foramen. All of the 

 abdominal vertebrae have a bony roof over the neural 

 arch and a single, undivided, neural spine. The haemal 

 arches and spines are well-developed and normal, except 

 for those of the first caudal vertebra, which has the 

 haemal spine single and undivided for the upper one- 

 third of its length, but bifid for the ventral two-thirds 

 of its length. The bifid portion of this spine overlies the 

 upper anterior surface of the first basal pterygiophore of 

 the anal fin. The haemal arch of the penultimate verte- 

 bra is not fused to its centrum, but, rather, is tightly 

 held to it by fibrous tissue. The haemal arch of the last 

 vertebra is described below. 



Caudal Skeleton. — The caudal skeleton consists of 

 the last vertebral centrum, one epural, two pairs of 

 uroneurals, five hypurals, and the parhypural, with the 

 whole complex receiving support from the prolonged 

 neural and haemal spines of the penultimate vertebra. 

 The last vertebral centrum is prolonged posterodorsally 

 into a urostyle and supports along its posterior edge the 

 five hypurals and the parhypural. From the dorso- 

 lateral surface on each side, the centrum bears antero- 

 dorsal projections which do not meet in the midline dor- 

 sally. These projections represent the lateral walls of the 

 neural arch of the last vertebra and its neural pre- 

 zygapophyses, since they form the lateral borders of the 

 neural canal and at their dorsal ends articulate by 

 fibrous tissue with the neural postzygapophyses of the 

 penultimate vertebra. The epural is a long rod articulat- 

 ing by fibrous tissue along its posterior edge with the 

 uroneurals and at its anteroventral edge with the area 

 between the dorsal ends of the two anterodorsal projec- 

 tions of the last vertebral centrum. The epural thus forms 

 the dorsal roof of the neural canal of the last vertebra and 

 it is, evidently, the very little modified neural spine of 

 the last vertebra. The uroneurals in this and the other 

 species of triacanthodids are highly variable in shape, 

 size, number and condition of pairing, as described in 

 detail by Tyler (1970b; see fig. 1 for illustrations of the 

 uroneurals in six specimens of P. lineata). There 

 is usually clear evidence of two pairs of uroneurals, 

 with the first pair usually smaller and more variable 

 than the second, more posterior, pair. In the caudal 

 skeleton of P. lineata illustrated here, the first pair 

 of uroneurals is represented by a single small rounded 

 plate above the anterior end of the second uroneural, 

 it being obvious that either one half of the first uroneural 

 has failed to develop or that the two halves have fused 

 into one piece. One other study specimen also has a 

 single first uroneural ossification, but three others have a 



