PAIRED FIN GIRDLES. 



Pectoral Fin. 



Supracleithrum. — In position at about a 45° angle 

 to the axis of the body; a massive straight shaft, very 

 thick dorsally but thinner ventrally. At its wide dorsal 

 edge the supracleithrum articulates by fibrous tissue 

 with the ventral surface of the posterolateral portion of 

 the pterotic, while anterodorsally the supracleithrum 

 abuts against the posterior edge of the posteroventral 

 flange of the pterotic and articulates with it by fibrous 

 tissue and, in the smaller specimen, by interdigitation, 

 the supracleithrum being immovably held to the 

 pterotic. Ventrally the supracleithrum broadly overlies 

 the cleithrum and anterior end of the dorsal post- 

 cleithrum, to both of which bones it is firmly held by fi- 

 brous tissue. There is no evidence in either of the two 

 study specimens of a posttemporal bone intervening 

 between the supracleithrum and pterotic, even as a small 

 bone extensively interdigitated or partially fused to one 

 of them. It would be of interest to know whether young 

 specimens of this species, the most generalized of the Re- 

 cent gymnodonts, give evidence to the fate of the post- 

 temporal in gymnodonts. 



Cleithrum. — Extremely long, reaching anteriorly to 

 between the dentaries in the lower jaw; laterally expand- 

 ed along most of its length but expanded posteriorly only 

 to a very slight extent in the area of the scapula; articu- 

 lates by fibrous tissue dorsolaterally with the overlying 

 supracleithrum and posterodorsally with the anterior 

 edge of the dorsal postcleithrum. Along its upper medial 

 edge the cleithrum articulates by fibrous tissue with the 

 anterior edge of the dorsal postcleithrum. Along its upper 

 medial edge the cleithrum articulates by fibrous tissue 

 with the anterior edge of the scapula and the anterodor- 

 sal edge of the coracoid. The ventral end of the coracoid 

 also articulates by fibrous tissue with the medial edge of 

 the cleithrum about one-third the way back the length of 

 the cleithrum. At their extreme anterior ends the medial 

 surfaces of the two cleithra are firmly bound to one 

 another by fibrous tissue, while just posterior to this 

 region the medial surfaces of the two cleithra support the 

 anterior end of the pelvis. 



Postcleithra. —The postcleithra form a long strut 

 from the ventral end of the supracleithrum along the ab- 

 dominal wall musculature to about one-third the way 

 back the length of the abdominal cavity. The dorsal post- 

 cleithrum articulates by fibrous tissue anteriorly with 

 the posterodorsal edge of the cleithrum and antero- 

 laterally with the posteromedial surface of the overlying 

 supracleithrum. Along nearly all of its ventral edge the 

 dorsal postcleithrum articulates by fibrous tissue with 

 the dorsal edge of the anterior half of the ventral post- 

 cleithrum, which is a somewhat slimmer shaft than the 

 dorsal postcleithrum. 



Coracoid. — Rounded anterodorsally, but tapering 

 to a slender shaft in the ventral half of ito length, a large 



posterodorsally directed process present from the poste- 

 rior edge of the rounded portion of the coracoid; cartilage 

 filled along the anterior edge of its rounded portion; ar- 

 ticulates by fibrous tissue anteriorly with the cleithrum 

 and posterodorsally with the third and fourth actinosta; 

 anterodorsally it articulates through cartilage with the 

 scapula. 



Scapula. — Scapular foramen entirely enclosed by 

 the scapula; cartilage filled at its anterior and ventral 

 edges; articulates by fibrous tissue anteriorly with the 

 cleithrum and ventrally through cartilage with the cor- 

 acoid. Along the upper half of its posterior edge the 

 scapula possesses two slightly protruding articular 

 facets, the more dorsal for articulation with the first fin 

 ray and the other for articulation with the ventral edge of 

 the first actinost. At the bottom of its posterior edge the 

 scapula supports the second actinost. 



Actinosts. — Four elements; all cartilage filled at 

 both ends; first actinost by far the smallest of the 

 elements and articulated by fibrous tissue with the up- 

 raised area on the posterodorsal edge of the scapula; sec- 

 ond, third, and fourth actinosts much larger than the 

 first and articulated by fibrous tissue with the postero- 

 ventral edge of the scapula in the case of the second ac- 

 tinost and to the dorsal edge of the coracoid in the case of 

 the third and fourth actinosts. Distally the actinosts sup- 

 port through fibrous tissue all of the pectoral fin rays, ex- 

 cept for the small first fin ray, which articulates with the 

 scapula. 



Fin rays. — Fifteen fin rays present on both sides of 

 the smaller specimen and 16 on both sides of the larger; 

 the first ray short, composed of two distinct halves 

 throughout its length, the medial half being by far the 

 stouter of the two; first ray articulated with the scapula, 

 but the other rays articulated with the actinosts. The 

 first two rays and the last ray unbranched, the other rays 

 branched; first ray without cross-striations, the other 

 rays cross-striated. 



Pelvic Fin. 



Pelvis.— An extremely long and stout shaft; its 

 posterior half tapering to a thin rod, while its anterior 

 half becomes ventrolaterally expanded and then tapers 

 to a bluntly rounded anterior end; the two halves of the 

 pelvis firmly interdigitated with one another, except at 

 the narrow posterior end where the interdigitation 

 becomes so extensive that distinct halves can no longer 

 be recognized and where fusion of the medial surfaces of 

 each half may have taken place. The ventral surface of 

 the anterior half of the pelvis is deeply concave and 

 houses the muscle mass whose contraction causes the 

 downward and forward rotation of the pelvis around its 

 anterior articulation with the cleithra in the process of 

 expansion of the huge ventral flap or dewlap. A small gap 

 is present in the otherwise interdigitated medial surfaces 

 of the two halves at a point just behind the middle of the 



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