OF THE SKULL IN SHARKS AND SKATES. 205 



apparatus of the upper jaw and palate of a Selachian should be developed out of a pair 

 o{ processes growing forwards from the primordial mandible, seems at first sight a most 

 unlikely thing ; and yet no fact in morphology has been better established. 



The clue to its discovery was the peculiar segmentation of the primary hyoid arch in 

 most vertebrates, either very obliquely through the upper third or half, or even, as in 

 the Salmon, fairly from top to bottom longitudinally, the upper segment in this case 

 generally growng forwards as a conjugational bar, yoking the hyoid on to the man- 

 dibular arch. Here, in the Shark, the primary and the secondary arches of the man- 

 dible (PI. XXXVI. fig. 4) are extremely alike both in form and size, and are specialized 

 to form this kind of mouth very perfectly. The halves of both meet at the middle, and 

 are conjoined by a strong ligament; sinuous, flat, and selvedged for the greater part of 

 their extent, they become terete and incurved distally. The articular region is gently 

 scooped for the quadrate condyle ; this is better seen in the side view of the adult 

 skull (PI. XXXVIII. fig. 2). 



The very simple hyoid arch (PI. XXXVI. figs. 3 & 4, h.m, c.liy, h.hy) is a solid struc- 

 ture ; the suspensorial part is morphologically a whole " epihyal " piece, with no pha- 

 ryngohyal segment above, and articulating with a " ceratohyal " from which no " hypo- 

 hyal " element has been cloven ; the right and left bars meet by the intervention of a 

 very elegant flat broad keystone piece, the " basihyal " {b.hy), which has a convex anterior 

 and concave posterior margin. 



In this extremely mobile face only one more keystone element is found, namely 

 the " basibranchial ; " and this belongs to the last two arches. The branchial arches, 

 five in number, are very uniform on the whole ; and the great development and singular 

 uniformity of these arches in the " Elasmobranchs " makes them fairly typical as to 

 the segmentation of this class of arches throughout the Ichthyopsida. Although the 

 basal piece is only developed in relation to the two hindermost arches, yet the two 

 in front of these have their distal symmetrical segment attached to that piece; thus, 

 functionally, it serves for four arches. Each arch has three fibrous joints on each 

 side, developed in the original substance of the joint by a limited production of con- 

 nective fibrous tissue instead of hyaline cartilage. Each bar, in its primary form, is 

 very elegant : it is bowed out laterally ; it descends in a sloping manner forwards ; and 

 both its apex and its distal end are hooked backwards, these hooks becoming the 

 " pharyngobranchial " and the " hypobranchial " elements (Plate XXXVI. figs. 3 & 4, 

 jp.Sr, h.br). The upper third of the main bar, answering to the hyoid suspensorial piece, 

 becomes the " epibranchial " {e.lr) ; and the remainder is the proper ceratobranchial 

 [c.hr). 



In the first branchial the hypobranchial is a small nucleus cloven from the anterior 

 lobe of the foot-shaped extremity (fig. 4, h.br, 1) ; but in the rest of the functional 

 arches the hypobranchial is a typical segment — large, free, and retrally attached to the 

 base of the ceratobranchial. 



VOL. X.— PART IV. No. o.— March 1st, 1878. 2g 



