RAJIFORMES 



155 



the jaws fuse together across, the palato-basal articulation being lost. 

 The attachment is hyostylic. The hyoid arch, however, becomes 

 peculiarly modified ; for, while the hyomandibular is specialised to 

 support the jaws only, losing its branchial rays, the remainder of 

 the arch becomes separately attached to the skull by a ligament 

 and a small epihyal {Raja, Fig. 99). The basal elements of the 

 branchial arches come to form a slender anterior transverse bar, and 

 a large posterior plate (Fig. 120). The last (5th) branchial arch be- 

 comes firmly attached to the pectoral girdle behind (Fig. 125). 

 Along the dorsal edge of the vertebral column large median 

 cartilages are often present ; they appear to be true neural spines 

 (p. 105), and accordingly the radials of the dorsal fins (which are 



Fi<i. 11s. 



Pectoral girdle and portion of the vertebral column of Raja hi, twin, Holt; oblique left- 

 side view, or, articular facets for pectoral fin ; c, coracoid region ; d.p, dorsal plate ; /, foramen ; 

 li, ligament ; sc, scapular region ; v.e, vertebral column. 



much concentrated), when present, reach down to the neural arches 

 (Fig. 49). _ 



The ribs tend to disappear. In the depressed forms the 

 ventral surface of the body becomes smooth and white, while the 

 dorsal surface, on the contrary, may be highly coloured and armed 

 with large sharp denticles. 



Almost all the divergences mentioned above from the normal 

 and more primitive type of Selachian structure may be directly 

 correlated with the habit of swimming, not with the tail, but with 

 the pectoral fins, and of living on the sea-bottom. 



Tribe 1. 



The large pectoral fins are produced forwards, but not fused 

 to the head. The branchial slits are partly ventral, partly lateral, 



