PHYLUM CHORDATA 



151 



considerable degree of uniformity. The main part of the expanse 

 of the fin is supported by a series of flattened segmented rods, the 

 pterygiopTiores or cartilaginous fin-rays, which He in close apposition : 

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

 the outer ends of these are one or more rows of polj^gonal plates of 

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

 number of slender " horny " rays or cerato-trichia of dermal 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 hdsal 

 cartilages articu- 

 lating with the 

 -pectoral arch. The 

 latter (pect.) is a 

 strong hoop of 

 cartilage incom- 

 plete dorsally, 

 situated immedi- 

 ately behind the 

 last of the 

 branchial arches. 

 It consists of a 

 dorsal, or scapu- 

 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, propterygitim (pro.), the middle, 

 mesopterygium (meso.), and the posterior, meiapteryghim (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 homy, they do not consist of true horn 

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

 characteristic of elastic connective-tissue fibres. 



Fig. 815. — Hemiscyllium, pefitoral arch find fin. d, r. dermal 

 horny rays ; meso. mesopterygiTim ; meta. nietapteryg:ium ; jtect. 

 pectoral arch ; pro. propterygium. , 



