XIII 



PHYLT\M riTORDATA 



151 



considerable degree of unifonnit}^ The main part of the expanse 

 of the fin is suj)ported by a series of flattened segmented rods, the 

 ptxrygio'phores or cartilaginous fin-rays, which lie in close ap])()sition : 

 in the case of the dorsal fins these may be partly calcified. At 

 the outer ends of these are one or more 1*0 ws of pol^-gonal plates of 

 cartilage. On each side of the rays and polygonal cartilages arc a 

 ninnber of slender " horny " rays or ccrato-trichia of der'mal origin.^ 

 In the smaller median fins there may be an elongated rod of 

 cartilage constituting the skeleton, or cartilage may be entirely 

 absent. In the pectoral fin (Fig. 815) the fin-rays are supported 

 on three hasal 

 cartiUujes articu- 

 lating with the 

 pcctural arch. The 

 latter (ped.) is a 

 strong hoop of 

 cartilage incom- 

 plete dorsal ly, 

 situated immedi- 

 atel}' behind the 

 last of the 

 branchial arches. 

 It consists of a 

 dorsal, or sccvpn- 

 lar, and a ventral, 

 or coracoid por- 

 tion, the coracoid 

 portions of oppo- 

 site sides being 

 completely con- 

 tinuous across the 

 middle line, while 

 the scapular are 

 separated by a 

 wide gap in which 



the spinal column lies. Between the two portions are the three 

 articular surfaces for the three basal cartilages. The coracoid 

 portions are produced forwards in the middle line into a flattened 

 process supporting the floor of the pericardial cavity in which 

 the heart is lodged. The three basal cartilages of the fin are 

 named, respectively, the anterior, propterygium (pro.), the middle, 

 mesopterygium (meso.), and the ])ostenor,_ meiapterygium (meta.). 

 Of these the first is the smallest and the last the largest : the first 



1 Though, on account of their appearance and horn-like consistency, these 

 structures are commonly referred to as horny, they do not consist of true horn 

 (which is always epidermal in origin), but of a substance called ehistin, 

 characteristic of elastic connective-tissue fibres, 



Fifi. 81.'). — HemiscyUium, peotimil arch and fin. d. r. denna 

 horny r;iys : lu'xd. mc-si.iiiteryghini ; met((. nietaptorygiinu ; peel. 

 pectoral aricli ; jjro. propterygium. 



