THE SKELETON OF THE DOGFISH. MEDIAN FINS. 79 



branchials are found elongated curved rods, the extra- 

 branchials (fig. 6, 19). The epi- branchials and cerato- 

 branchials bear gill rays along their posterior borders. 



C. THE SKELETON OF THE MEDIAN FINS. 



The dorsal fins have a skeleton consisting of a series of 

 short cartilaginous rods, the basals or basalia, which slope 

 obliquely backwards. Their bases are imbedded in the 

 muscles of the back, while their free ends bear a number 

 of small polygonal cartilaginous plates, the radials or radialia. 

 Associated with this cartilaginous skeleton are a number of 

 long slender horny fibres, the fin-rays, which have been already 

 referred to in connection with the exoskeleton. The skeleton 

 of the other median fins mainly consists of these fibres, the 

 cartilaginous portion being reduced or absent. 



2. THE APPENDICULAR SKELETON. 



This includes the skeleton of the two pairs of limbs and of 

 their respective girdles. 



THE PECTORAL GIRDLE forms a crescent-shaped hoop of 

 cartilage, incomplete above and lying just behind the visceral 

 skeleton. The midventral part of the hoop is the thinnest 

 portion, and is drawn out -in front into a short rounded process 

 which is cupped dorsally and supports part of the floor of the 

 pericardium (tig. 7, 1). On each side of this flattened mid- 

 ventral portion the arch becomes very thick and bears on its 

 outer border a surface with which the three basal cartilages 

 of the fin articulate. The dorsal ends or scapular portions 

 of the girdle form a pair of gradually tapering horns. 



THE PECTORAL FIN articulates with the pectoral girdle by 

 means of three basalia or basal cartilages, the pro-pterygium, 

 meso-pterygium and meta-pterygium. The most anterior 

 and the smallest of these is the pro-pterygium (fig. 7, 5), 



