A treatise on zoology . elements,was supposed to represent the original axis. Vestiges of postaxialrays are occasionally found (Fig. 53). The metapterygium was heldby Gegenbaur not to be formed by concrescenT;e. This distinction, drawn between the posterior and largest basal(the metapterygium) and the others, does not seem to be justifiedeither by comparative anatomy or by embryology. Indeed, Huxley[230] identified the original axis in the mesopterygium. All thebasals are probably formed by concrescence, and an endless variety 128 ELA SiUO BRA NCHII of detail is presented liy the difterent fam
A treatise on zoology . elements,was supposed to represent the original axis. Vestiges of postaxialrays are occasionally found (Fig. 53). The metapterygium was heldby Gegenbaur not to be formed by concrescenT;e. This distinction, drawn between the posterior and largest basal(the metapterygium) and the others, does not seem to be justifiedeither by comparative anatomy or by embryology. Indeed, Huxley[230] identified the original axis in the mesopterygium. All thebasals are probably formed by concrescence, and an endless variety 128 ELA SiUO BRA NCHII of detail is presented liy the difterent families and genera. Theremay be a single basal, as in Sci/mnus (Fig. 53, A); two basals, as inHeterodontus and Chimaera (Fig. 54, A); three, as in Sci/llium orAcanthias ; or five, as in Myliohatis. In the Eajidae, also, an anterioraxis develops quite similar to the metapterygial axis (Fig. 121). The pelvic fin is much less completely constricted off than thepectoral; its skeleton is simpler. In the Selachii, the fanlike B. Fio. 87. Skeleton of the pectoral arch and fln of A, Cladothis Neilsoni, Traq. ; B, Chlamydoselaclnisangxiineus, Gannan ; and 0, Symmorium reniforme, Copp. (A restored from Traquairs figure, Band C after Brans.) c, coracoid region ; , distal basal or 3rd segment of metapterygial axis ;, diazonal nerve foramen ; /, problematical fin outline ; j, joint between girdle and fin ; ws,niesopterygium ; mt, metapterygium ; n, nerve foramen ; , posterior preaxial radial; pro,propterygium ; , anterior preaxial radial; , possibly postaxial radial ; s, scapular region ;ta, distal segment of metapterygial axis. arrangement of the radials, although to some extent visible atthe posterior end, is never as pronounced as in the pectoral for a iew radials which may articulate with the girdle,the single series is attached to one longitudinal basal, the basiptery-gium, lying in the body-wall. Postaxial rays, if ever present,have disappeared, leaving but
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