pterygiophore. From the lateral surface of the haemal 

 spine of the first caudal vertebra, just below the level of 

 the haemal arch, the seventh or last epipleural ar- 

 ticulates by fibrous tissue. The second caudal vertebra 

 has the largest haemal spine of any of the vertebrae, and 

 posterior to this vertebra the length of the haemal spines 

 progressively decreases until the 10th caudal vertebra is 

 reached. The 10th caudal vertebra has its haemal spine 

 longer than those just anterior to it, and in contrast to all 

 the others, its haemal spine is autogenous. The lengths of 

 the neural spines decrease from the first to the 10th 

 caudal vertebra. The 10th caudal vertebra has its neural 

 spine longer than those just anterior to it. The haemal 

 and neural spines of the last caudal vertebra are de- 

 scribed below. 



Caudal Skeleton. — The caudal complex consists of an 

 epural, a free upper hypural, a free parhypural, and a 

 large plate composed of the centrum fused to most of the 

 hypural elements. The epural is a narrow shaft dorsally 

 but an expanded plate ventrally, where it articulates by 

 fibrous tissue with the neural arch of the centrum, some- 

 times more closely than that in the illustrated specimen. 

 The free upper hypural is wider dorsally than ventrally 

 and is held by fibrous tissue between the edges of the 

 epural and the posterior half of the dorsal edge of the fus- 

 ed hypural plate. The parhypural is a flattened rod 

 similarly held between the edges of the posteroventral 

 half of the autogenous haemal spine of the penultimate 

 vertebra and the ventral lobe of the fused hypural plate. 

 The large hypural plate is the centrum fused to what in 

 more generalized plectognaths are the first to fourth 

 hypurals, the deep indentation on the posterior edge of 

 the plate representing what would be the division 

 between the second and third hypurals. The centrum 

 region is thickened into a broad vertical crest for muscle 

 attachment. The two halves of the neural arch of the last 

 centrum diverge anteriorly, but more posteriorly they 

 sometimes approach each other more or less closely in 

 the midline, although they do not interdigitate or fuse. 

 The neural canal is thus relatively open above, never be- 

 ing more than very partially roofed over. Similarly, the 

 two halves of the haemal arch closely approach one 

 another ventromedially, but do not interdigitate or fuse 

 and leave the haemal canal only partially roofed over. 

 The haemal canal courses through the haemal arch to 

 exit at the deep notch in the anteroventral end of the fus- 

 ed hypural plate. The parhypural thus represents only 

 the haemal spine of the last vertebra, and not both the 

 spine and arch. 



As discussed and illustrated by Tyler (1970b:14, fig. 22) 

 three of the five study specimens show no evidence of 

 uroneurals, nor do any of the other species of balistids ex- 

 amined, but two of the specimens of Balistapus un- 

 dulatus do. In one of these specimens there is a pair of 

 small nubbins of bone resting on the dorsal surface of the 

 upper free hypural and directly behind the posterior edge 

 of the epural, these apparently being the two halves of a 

 rudimentary uroneural. In the other specimen, the "uro- 

 neural" elements are much further forward, being a pair 



of ossifications just above and behind the posterodorsal 

 region of the neural arch of the last centrum. Whether 

 these elements are homologous to the uroneurals of the 

 other specimen is questionable, for they could also be un- 

 consolidated neural arch material, as sometimes found in 

 triacanthodids, which have relatively well-developed 

 uroneurals. 



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

 most ray and the lowermost ray unbranched, the others 

 becoming increasingly branched toward the middle fin 

 rays, which are branched in triple dichotomies. The bifid 

 bases of the fin rays articulate by fibrous tissue with the 

 caudal skeleton as follows: the uppermost two rays to 

 the upper free hypural; the lowermost ray to the parhy- 

 pural and the remaining eight rays to the lobes of the 

 fused hypural plate, four to the dorsal lobe and four to 

 the ventral, with the lowermost branched ray receiving 

 partial support from the parhypural as well. 



DORSAL AND ANAL FINS. 



Spines and pterygiophores. — Three spines borne on 

 two basal pterygiophores. First spine long and stout; sec- 

 ond spine slightly shorter and narrower; third spine 

 about one-fourth the length of the first spine. The first 

 two spines are borne on the enlarged first pterygiophore 

 and the third spine on the much smaller second pteryg- 

 iophore. The mechanism by which the bifid bases of the 

 first two dorsal spines rotate over the medial upraised 

 portion of the dorsal surface of their pterygiophore and 

 lock themselves in an erected position, with the postero- 

 ventral edge of the first spine resting against the antero- 

 ventral edge of the second spine, has been described so 

 often in the past that a detailed redescription of it here 

 seems superfluous. For such descriptions see Hollard 

 (1853:102), Bnihl (1856:59; 1880; 1891:pl. 24), Mayer 

 (1864:fig. 4 of pi. 7), Bliss (1872:10), Thilo (1879:12; 

 1896b:291; 1902), Klein (1881:350), S(<rensen (1884:50; 

 1897), Mohr (1928), Jacobs (1935:156-157), Clothier 

 (1939), Smith (1949a:406), and Monod (1950, 1960). The 

 papers of S</rensen, Clothier, and Monod are par- 

 ticularly recommended. Anteriorly the first pterygio- 

 phore has a short, bluntly rounded, ventral process which 

 is firmly held by fibrous tissue in the concavity in the 

 midline of the posterodorsal region of the skull formed 

 dorsally by the supraoccipital and laterally and ven- 

 trally by the epiotics. Posteriorly the first pterygiophore 

 is broadly overlain by the anterior region of the second 

 pterygiophore, with which it interdigitates. On the ven- 

 tral edge of the second pterygiophore, there is a shallow 

 indentation where fibrous tissue articulation is establish- 

 ed with the thick shaftlike supraneural element. The 

 supraneural slants posteroventrally from its articulation 

 with the second pterygiophore to make fibrous tissue 

 contact with the dorsal edge of the neural spine of the 

 fifth abdominal vertebra and with the lower anterior 

 edge of the first basal pterygiophore of the soft dorsal fin. 



109 



