and slight interdigitation with the broadly overlying 

 supracleithrum and by fibrous tissue with the anterior 

 edge of the dorsal postcleithrum; posteriorly in about the 

 middle of its length it articulates by fibrous tissue and 

 slight interdigitation with the scapula and coracoid. Ven- 

 tromedially the cleithrum articulates by fibrous tissue 

 with its opposite member, while the dorsal edge of its 

 medially expanded platelike portion is held by fibrous 

 tissue to the ventral surface of Baudelot's ligament. 



Postcleithra. —The postcleithra form a thin wide 

 plate, closely held by fibrous tissue to the cuirass, from 

 the upper end of the cleithrum to about the level of the 

 posteriormost point of the pectoral girdle. The dorsal 

 postcleithrum is widest and thickest at its anterior edge 

 where it articulates by fibrous tissue with the cleithrum. 

 Posteriorly the dorsal postcleithrum articulates by fi- 

 brous tissue, and in large specimens by slight inter- 

 digitation, with the ventral postcleithrum. The ventral 

 postcleithrum is an extremely thin plate of variable 

 shape, but it is always much smaller than the dorsal 

 postcleithrum. The two postcleithra overlie one another 

 in a variable manner at their region of articulation. 



Coracoid. — Wider ventrally than dorsally; its pos- 

 terior edge with a laterally directed flange throughout its 

 length, except at the extreme dorsal and ventral ends; a 

 short dorsal projection present from its posterodorsal 

 edge which makes contact with the posterior edge of the 

 last actinost; cartilage filled at its dorsal edge; an up- 

 raised flange present on its lateral surface, running, with 

 increasing height of the flange, from posterodorsally to 

 anteroventrally; articulates dorsally through cartilage 

 with the scapula, while posterodorsally its small dor- 

 sally directed flange interdigitates with the posterior 

 edge of the last actinost. Dorsally along its anterior end 

 the coracoid articulates by fibrous tissue and slight in- 

 terdigitation with the cleithrum, which broadly overlies 

 it. 



Scapula. — Completely encloses the scapular 

 foramen; cartilage filled at its ventral and posterior 

 edges; articulates by fibrous tissue and slight inter- 

 digitation anteriorly and anteroventrally with the cleith- 

 rum, and through cartilage ventrally with the coracoid; 

 posterodorsally the scapula interdigitates with the bases 

 of the first two actinosts, while more anteriorly on its dor- 

 sal edge it bears a concavity to which the reduced first 

 pectoral fin ray articulates by fibrous tissue. 



Actinosts. — Four elements; all cartilage filled at 

 their dorsal 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 by fibrous tissue with the scapula 



and coracoid; fourth actinost articulated by fibrous tis- 

 sue ventrally with the coracoid, as well as by inter- 

 digitation posteriorly with the dorsally directed process 

 of the coracoid. Distally the actinosts support by fibrous 

 tissue all of the fin rays, except for the first, which is sup- 

 ported by the scapula, and the second, which is sup- 

 ported by both the scapula and first actinost. 



Fin rays. — Twelve fin rays in most specimens, with 

 the first ray about one-sixth the length of the second ray 

 and articulated directly with the scapula instead of with 

 the actinosts, as are all the other rays, except for the sec- 

 ond, which articulates with the region of interdigitation 

 between the scapula and first actinost; first ray with its 

 two halves fused together, the division between the two 

 halves being indicated only by an oblong cavity running 

 horizontally through the base of the ray; the medial half 

 of the ray larger than the lateral half. The first two rays 

 unbranched, the others branched. The first ray without 

 cross-striations, the other rays cross-striated. 



VERTEBRAL COLUMN. —All vertebrae with bicon- 

 cave centra, except the last, which ends posteriorly in the 

 fused hypural plate, and the highly modified first five 

 vertebrae. 



Abdominal Vertebrae. 



First five vertebrae. — What has usually been called 

 the first vertebra is in fact a compound element resulting 

 from the fusion of the first five abdominal vertebrae to 

 themselves and to the posterior end of the basioccipital 

 (as described in detail by Tyler 1963a). In adult 

 specimens there is slight evidence of this fusion of the 

 first five vertebrae into a single piece, for there are five 

 neural foramina in the upper half of the lateral surface of 

 the fusion product and its neural arch shows, except in 

 extremely large specimens, a division or lobation dor- 

 sally into five successive regions separated from one 

 another by clefts which extend downward about one- 

 third to one-half the distance from the top of the neural 

 arch to the region of the centrum. In the 8.2 mm 

 specimen each of the last three of the first five vertebrae 

 possesses a very thin centrum and neural arch in a single 

 continuous piece separate from that succeeding and 

 preceding it. The first and second vertebrae are even 

 thinner than the others and their centra are in close ap- 

 position and may even be fused at this early develop- 

 mental stage. These two centra, in turn, appear to be in 

 the process of fusing with the basioccipital, but their 

 neural arches are still separate from one another. In the 

 15.3 mm specimen the first two vertebrae have their cen- 

 tra and the lower one-fourth of their neural arches indis- 

 tinguishably fused, while the centra are similarly fused 

 to the basioccipital. The third and fourth vertebrae have 

 their centra and the lower three-fourths of their neural 

 arches indistinguishably fused, but separate from the 

 fused first two vertebrae and the fifth vertebra. In the 



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