OF THE SKULL IN SHARKS ADD SKATES. 211 



On the ceratohyal (PI. XXXVIII. figs. 1 & 2, c.hy, hr.r) there are seven, the upper 

 of these being quinquefid, and the rest bifid. The branchial rays on the next four 

 arches (the principal branchials) are simple in form, and range in number from ten 

 on the first to five on the last. Tv?o or three of these are on the " epibranchial," the 

 counterpart of the hyomandibular, and the rest on the ceratobranchial." 



The hyoid arch meeting with the chondrocrauium, and articulating veith it, has no 

 " pharyngo-pleural " element, a part developed in all the succeeding bars, which float, 

 as it were, over the large pharyngeal cavity, and are not attached to the axis of the 

 animal. 



The branchial arches (PL XXXVII. fig. 2, br, and PI. XXXVIII. figs. 1 & 2, br) are, 

 on the whole, very uniform ; but the fifth, or last, is abortively developed ; on it, as on 

 the hyoid, there is but one series of branchial plicae. These arches have great mobility, 

 and form complete girdles to the huge pharynx ; their apices meet together above, and 

 their distal parts below ; behind, there is a large keystone piece to the last two of the 

 arches (PI. XXXVII. fig. 2, and PI. XXXVIII. figs. I & 2, b.br). Having great mo- 

 bility, the dorsal segments (pharyngo-branchials, ^.5;') may turn forward (PI. XXXVII. 

 fig. 3, and PI. XXXVIII. fig. 2), or backwards (PI. XXXVIlf. fig. 1) ; normally they 

 turn backwards i, the opposite direction to that taken by the hyoid and mandibular 

 arches. Nevertheless the segment that corresponds with the upper part of those 

 arches, namely the epibranchial {e.br), sends forward a 'pedate process, the exact coun- 

 terpart of the fore part of the head of the hyomandibular, and of the quadrato- 

 pterygoid lobe of the mandibular arch. The pharyngobranchials are thus seen to be 

 attached behind the pedate process, which is what occurs in the hyoid of the frog, 

 where the pharyngohyal element (" interstapedial ") is developed on the end of the 

 hinder fork of the bar. Primarily the direction of the dorsal ends of the visceral 

 arches is inwards and backwards, and the foreturned part is a secondary spur or fork ; 

 the ventral or lower ends have also a retral habit of growth ; so that normally the hypo- 

 pleural element is always more or less hooked backwards. In the first branchial 

 of the Dog-fish the hypobranchial is merely a bud, segmented off from the anterior 

 lobe of the pedate lower end of the arch. In the rest of the principal arches [h.br 2, 

 3, 4) this ventral element is a long terete rod, sharply retral, and forming an acute 

 angle with its fellow of the opposite side: this segment does not exist on the last 

 arch ; and its pharyngobranchial is not distinct from that of the fourth branchial 

 (PI. XXXVII. fig. 2, and PI. XXXVIII. fig. 2). 



The epibranchial and ceratobranchial series are closely like the " epi- and cerato- 

 hyals," save that they are smaller; they are similarly palisaded with branchial rays. In 

 the lower view (PI. XXXVIII. fig. 1) the seriality of these arches is well seen. The 

 hyoid arch, with its multifid branchial rays, that take the place of the bony opercular 

 and subopercular of the Osseous Fish, is the only arch, besides the last branchial, that 

 ' I have figured them in both ways for illuatration. 



