THE ELASMOBRANCH FISHES 



49 



observed that to each segment two neural arches are present, and that a similar 

 condition obtains in tlie haemal arches (h.a.) even farther forward. Such a 

 condition is incomplete diplospondyly. 



Appendicular Skeleton 



The appendicular skeleton is the framework for the fins and the girdles to 

 which these, if paired, are attached. 



The pectoral girdle (fig. 54) in Heptanchus is a slender arch open dorsally, 

 to which the framework of the pectoral fin is attached. It is composed of right 



ms.p. 



•t.pf 



Fig. 54. Lateral view of the skeleton of the pectoral fin and girdle, Heptanchus maculaius. 

 (Duncan Dunning, del.) 



a.pl., process for articulation of pectoral fin; co., coracoid; f-pi., foramen for nerves and 

 blood vessel; ms.p., niesopterygium ; mt.p., metapterygium ; pr.p., propterygium ; ra., ra- 

 dials; sc, scapula. 



and left cartilaginous halves which are united in the middle line below by 

 means of an unpaired median piece. The part of the girdle extending the more 

 dorsalward is the scapula (sc), tipped by the suprascapula; that part which, 

 by means of the median piece, joins a similar part from the opposite side 

 below is the coracoid portion (co.). At the middle and posterolateral part of 

 each half of the girdle there is an irregular surface for articulation with the 

 pectoral fin (a.pl.). In front of and below this projection is a broad surface 

 for the attachment of the ventral muscles of the fin. Perforating the girdles in 

 this surface is a large foramen (f.pt.) through which the lirachial artery and 

 nerves pass to supply the fin. 



