dorsally to anteroventrally where it contacts the pos- 

 teroventral end of the cleithrum; articulates dorsally 

 through cartilage with the scapula, while posterodor- 

 sally it articulates through cartilage (and interdigitation 

 in one of the two study specimens) with the third actinost 

 and through cartilage and interdigitation with the fourth 

 actinost. It is probable that in large specimens the third 

 actinost would also interdigitate with the coracoid. 

 Anteriorly the coracoid articulates through cartilage and 

 in some places slight interdigitation with the posterior 

 edge of the cleithrum. 



Scapula. — Completely encloses the scapular 

 foramen; cartilage filled at its posteroventral and 

 anterior edges; articulates by fibrous tissue and inter- 

 digitation anteriorly and anteroventrally with the 

 cleithrum, while posteroventrally it articulates through 

 cartilage with the coracoid. Posterodorsally the scapula 

 articulates through fibrous tissue and interdigitation 

 with the bases of the first two actinosts and, in one of the 

 two study specimens, with the anterior end of the base of 

 the third actinost. Just anterior to its articulation with 

 the first actinost the scapula bears a projection with a 

 concave surface for support of the short uppermost pec- 

 toral fin ray, to which it is held by fibrous tissue. 



Actinosts. — Four elements; cartilage filled at their 

 ventral edges; first actinost the smallest, the others of 

 slightly increasing size posteriorly in the series; all four 

 elements held to one another by interdigitation; first and 

 second actinosts articulated ventrally by interdigitation 

 with the posterodorsal edge of the scapula; third actinost 

 articulated ventrally either through cartilage or by inter- 

 digitation with the dorsal edge of the coracoid; fourth ac- 

 tinost interdigitated with the posterodorsal edge of the 

 coracoid. Distally the actinosts support by fibrous tissue 

 all of the fin rays, except for the first, which is supported 

 by the scapula, and perhaps the second, which may be 

 supported in part by the scapula as well as by the first 

 actinost. 



Fin rays. — Twelve to thirteen fin rays in most 

 specimens, with the first ray only about one-fourth or 

 one-fifth the length of the second ray and articulated 

 directly with the scapula instead of with the actinosts, as 

 are all the other rays with the possible exception of the 

 second, which articulates more or less in the region of in- 

 terdigitation between the scapula and first actinost; first 

 ray with its two halves distinct and of about equal size. 

 The first two rays unbranched, the others branched. The 

 first ray has cross-striations at its extreme distal end and 

 the other rays are more fully cross-striated. 



VERTEBRAL COLUMN. —All vertebrae with 

 biconcave centra, except the last, which ends posteriorly 

 in the fused hypural plate, and the highly modified first 

 two vertebrae. However, the third and fourth vertebrae 

 particularly are much compressed anteroposteriorly and 

 the biconcaveness of the centra is only slightly indicated. 



Abdominal Vertebrae. 



First two vertebrae. — In the two adult study 

 specimens, the first two abdominal vertebrae are 

 rudimentary and are not only fused to one another and to 

 the skull but are much smaller than any of the subse- 

 quent vertebrae. The neural arches of the first two 

 vertebrae are slender and the arches from each side do 

 not meet their opposite members in the midline above 

 the spinal cord. They are distinct from one another in the 

 dorsal halves of their lengths and are fused to one 

 another and to the exoccipitals and basioccipital only 

 basally. The fused centra of the first two vertebrae are so 

 compressed anteroposteriorly and shortened dorsoven- 

 trally that they form only a thin plate fused to the dorsal 

 region of the posterior end of the basioccipital. The 

 neural arches of both vertebrae bear small neural 

 foramens. The structure of the first two vertebrae and of 

 the vertebral column in general has been described by 

 Tyler (1963a:164-171). 



Other abdominal vertebrae. — As used here the term 

 abdominal vertebrae refers to all of those vertebrae 

 anterior to the vertebra to which is attached the first anal 

 fin basal pterygiophore. In the case of ostracioids, with 

 their highly unusual anal fin basal pterygiophore ar- 

 rangement, this definition is not applicable. The more 

 anterior of the anal fin basal pterygiophores of os- 

 tracioids do not remain in the midvertical plane of the 

 body, but, rather, are laterally divergent from their 

 medially placed bases and thus no longer connected with 

 the ventral surface of the vertebral column. Only the last 

 three or four basal pterygiophores are invariably in the 

 midline of the body, and the proximal ends of even these 

 pterygiophores in ostracioids do not make as close a con- 

 tact with the vertebrae as in other plectognaths. Under 

 these circumstances (discussed more fully by Tyler 

 1963a: 169) it is necessary to arbitrarily state that the first 

 caudal vertebra is that vertebra to which the proximal 

 end of the first basal pterygiophore that consistently lies 

 entirely in the midline of the body is most closely 

 associated. Although this divides the vertebral column 

 into abdominal and caudal segments that are not com- 

 parable to those of other plectognaths, it has consistency 

 within the superfamily, for all specimens of ostracioids 

 examined have, by this definition, 9 abdominal 

 vertebrae, regardless of how many of the anterior 

 elements are fused together, and either 9 (usually) or 10 

 caudal vertebrae. In Kentrocapros aculeatus the first 

 basal pterygiophore of the anal fin that lies entirely in 

 the midline of the body is supported or most closely as- 

 .sociated with the sequentially 10th vertebra, which ar- 

 bitrarily is thus defined as the first caudal vertebra. The 

 third to ninth abdominal vertebrae have normal, un- 

 fused, centra, and complete neural arches with well-de- 

 veloped neural spines. These abdominal vertebrae differ 

 from the more anterior of the caudal vertebrae only in 

 that the first few vertebrae following the two rudimen- 

 tary vertebrae (i.e., the third to about the sixth 



