146 



ZOOLOGY 



SECT. 



the outer ends of these are one or more rows of polygonal plates of 

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

 number of slender " horny " rays or ceratotrichia of dermal origin. 1 

 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. 825) the fin-rays are supported 

 on three basal cartilages articulating with the pectoral arch. The 

 latter (pect.) is a strong hoop of cartilage incomplete dorsally, 

 situated immediately behind the last of the branchial arches. 



mck 



hyp.br. 



ce,r.br.a 



ph.br. 3 



FIG. 824. Hemiscyllium, ventral view of the visceral arches. Letters as in preceding figure. 

 In addition b. br. basibranchial plate ; cer. br. ceratobranchials ; hyp. br. hypobranchials. 



It consists of a dorsal, or scapular, and a ventral, or coracoid portion, 

 the coracoid portions of opposite sides being completely continuous 

 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 



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 elastin, 

 characteristic of elastic connective -tissue fibres. 



