214 ME. W. K. PAEKER ON THE STEUCTUEE AND DEVELOPjNIENT 



all the bars, except the first or spiracle, large, spatulate, external branchial filaments 

 growl, six or seven on an average to each bar on each side; they are now, in the 

 longest embryo, the largest of them, as long as the head; each contains, as in the 

 embryo of the Dog-fish, a single capillary loop. Four short buds appear in the 

 "spiracle," or first postoral cleft (PI. XXXV. fig. 1, ell): in embryos that are evident 

 Skates a large pseudo-branchia is seen inside the front wall of the spiracle, and arising 

 from the back of the proper apex of the mandible (PI. XL. fig. 3, c^ 1, ps.hr). 



The branchials {br 1-5) are simple rods; not so the rudimentai'y mandibular and 

 hyoid arches ; these are bifoliate above, more markedly bifurcate than in the Dog-fish, 

 in which they were pedate (PI. XXXIV.). 



Here the bifurcations are filled with granular substance, which becomes solid cartilage 

 in each fork ; and in this the Skate comes nearer to the ganoid and teleostean fishes. 



In each case the hinder spur is slenderer than the front ; and in the mandibular arch 

 it is seen that the hinder hooked snag is but loosely connected with the main part. 

 This is in front of the "spiracle" {el 1); but, behind this opening, the front part of 

 the next bar is severed from the long hinder stalk. 



The front fork of the mandibular arch turns downwards and forwards to become the 

 upper jaw ; the small backwardly curved spike, the hinder fork, becomes the spiracular 

 cartilage. 



In the second arch (hyoid) the large front fork becomes the hyo-mandibular, and 

 the rest of the bar the proper hyoid. A reference to the figures will show that the 

 suspensorium of the mandible, the hinder fork, although it embraces the anterior 

 spiracular lip, is yet attached to the head below and behind the exit of the trigeminal 

 nerve, antero-inferiorly to the auditory sac; it is a. free "otic process." 



The anterior fork of the hyoid arch is in inferior relation to the auditory sac ; it 

 articulates broadly with it afterwards as the hyo-mandibular, whilst the hinder fork, or 

 " stylo-ceratohyal," is loosely connected with that sac behind. In the various types, and 

 even in the various stages of one and the same type, the hinder division of the second 

 postoral is very variously articulated to the surrounding parts. 



It is worthy of remark that that which distinguishes the Rays from the Sharks most 

 completely, namely the mode of segmentation of the hyoid arch, is already evident in 

 the embryos at this stage. The Skate breaks up this arch in the same manner as 

 the Sauropsida and the Mammalia ; whilst the Shark shows it in a simple and low 

 form. I have worked out the basis cranii in the youngest Pristiunis, f of an inch 

 long, and the visceral arches in the larger specimen of Pristiunis, f of an inch long 

 (PI. XXXV. figs. 3 & 4). 



When the base of the skull is seen from above (fig. 3) there are three intercapsular 

 regions displayed, although the bend of the head throws the eye-ball into the same 

 vertical line as the nasal sac (see fig. 1). If we suppose a curved line passing con- 



' They are small in the first cleft. 



