PECTORAL FIN. 



131 



somactids, which carry peripherally the dermotrichia.* There 

 are generally three basal somactids which articulate directly 

 with the pectoral girdle (Fig. 75, B) : these are called respective!}^ 

 the pro- meso- and meta-pterygium. They carry the peripheral 

 radialia. The metapterygium is the largest of the basal pieces 

 and carries the greatest number of peripheral somactids. These 

 are placed mainly on its preaxial side (Fig. 75), there being 

 few, if any, on its postaxial side. 



A ^ 



Fig. 75. — Skeleton of pectoral fin ^i of Scymnus, B of Acanthias vulgaris (after Gegen- 

 baur). p pro-, ms meso-, mt meta-pterygium ; B postaxial (median) side of fin. (The 

 line drawn through nU in B indicates what some anatomists regard as the axis of the 

 so-called archipterygium ; on the same view the dotted lines R, R indicate the preaxial 

 radii, R' the postaxial radii). 



The metapterygium appears to be the most important basal, and when 

 there is only one basal somactid, as in Scymnus (Fig. 75, A), it is supposed 

 to be the metapterygium. 



In living forms the fin-skeleton is always rhipidostichous (p. 57), but 

 in some extinct forms {Cladoselache, Fig. 83) it is orthostichoiLS, and in 

 others (Pleuracant)ms, Fig. 76) it is rachiostichous (and unibasal) and on 

 the whole plem?orachic (p. 57). In Raji the propterygiuiii and mesoptery- 

 givim are elongated and segmented, and the propterygium is attached to 

 the olfactory region of the skull. Moreover in the Raji there are sometimes 

 additional basals inserted between the meso- and meta-pterygimn. 



The pelvic girdle is not attached to the vertebral column ; 

 it consists of a transverse bar of cartilage placed just ventral to 



* Dermotrichia are absent from the paired fins of some Raji (e.g. Tor- 

 pedo, etc.). 



