128 



ELASMOBRANCHII 



of detail is presented by the different families and genera. There 

 may be a single basal, as in Scymnus (Fig. 53, A) ; two basals, as in 

 Heterodontus and Chimaera (Fig. 54, A) ; three, as in Scyllium or 

 Acanthias ; or five, as in Myliobatis. In the Rajidae, also, an anterior 

 axis develops quite similar to the metapterygial axis (Fig. 121). 



The pelvic fin is much less completely constricted off than the 

 pectoral ; its skeleton is simpler. In the Selachii, the fanlike 



B 



pro. 



7ns. 



[>rr. 



it. 



-pr. 



FIG. 87. 



Skeleton of the pectoral arch and tin of A, Chulodus Neilsoni, Traq. ; B, Chlamydostlaclnts 

 anguineus, Gannan ; and C, Symmorium remforne, Cope. (A restored from Traquair's figure, B 

 and C after Brans.) c, coracoid region ; d.b, distal basal or 3rd segment of metapterygial axis ; 

 d.n, diazonal nerve foramen ; /, problematical fin outline ; j, joint between girdle and tin ; ms, 

 mesopterygium ; mt, metapterygium ; TO, nerve foramen ; p.r, posterior preaxial radial ; pro, 

 propterygium ; pr.r, anterior preaxial radial ; pt.r, possibly postaxial radial ; s, scapular region ; 

 ta, distal segment of metapterygial axis. 



arrangement of the radials, although to some extent visible at 

 the posterior end, is never as pronounced as in the pectoral fin. 

 Except for a few 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 occasionally a doubtful remnant 

 (Figs. 96, 101). The pelvic fin is almost or quite monostichous. 



The articulation of radials to the pelvic girdle in front of the 

 metapterygium is doubtless a primitive character; it is an indication 



