OF THE SKULL IN SHAEKS AND SKATES. 217 



A diif&rence is to be noted at once ; for the spiracular cartilage is developed in the 

 posterior fork of the first visceral arch, and this nucleus in the anterior fork (PI. XXXV. 

 figs. 1 & 4). 



The mandibular nucleus looks outwards and backwards (s/i.c); but this is turned 

 directly forwards at right angles to the main bar (fig. 4, h.m, hy. Compare also here 

 the adult skull, PI. XLI. fig. 4, mtpg, km) ; the " metapterygoid " is the same as the 

 spiracular cartilage in the Skate). 



The main hyoid bar is a slender tape of cartilage, pointed finely at each end, and 

 far from the mid line at both ends ; it is undivided at present. 



The ne-atfive arches are the ordinary branchials : the first is much larger than the 

 hyoid ; and then they gradually lessen, the last being very small. 



These arches are strongly bent upon themselves, the pharynx of the embryo Skate 

 being a depressed pouch with extensive lateral fissures or clefts (figs. 1, 2 & 4). 



The main part of each arch is pointed above, the point looking slightly forwards, and 

 blunt below, this rounded lobe looking backwards; both ends come much nearer to the 

 mid line than in the hyoid arch. In the roof of the pharynx, where the front clefts 

 close there is, just above each main branchial, an independently chondrified piece — the 

 " pharyno'o-branchial " [p.br); each piece is lunate, pointed outside, blunt within, and 

 havin" its point turned more backwards than outwards, although their general direction 

 is transverse. 



These free cartilages cannot very safely be considered as the serial homologues of those 

 above the two first arches ; they are not at the side of the face, but right beneath the 

 edges of the parachordals, where the leashes of nerves are given oif (figs. 3 & 4). 



A separate cartilage, developed at the end of the pterygo-quadrate, or one above the 

 point of the main hyoid, would correspond more truly. 



The further segmentation and metamorphosis of the skull and its arches will now be 

 described. I have shown above what, from the beginning, was independently chondri- 

 fied, and now will show how the main bars break up. 



Second Stage : Embryo of Eaia maculata, 4 inches long ; body 1^ in., tail 2f in. ; 

 time from deposit of egg-pouch 3 months. 



This important specimen was taken for me from the Brighton tank by the same 

 valued friend, Mr. H. Lee ; and although the time of its growth was less than twice 

 that of the early specimen, the development and metamorphosis was quite perfect — 

 that is, as to chondrification and segmentation. 



There is much that is instructive to the morphologist in the external characters of 

 this embryo (PI. XL. figs. 1 & 2, drawn as far as to the umbilicus, u). In front the 

 beak has become fixed to the anterior angle of the outspread, gigantic, flabelliform 

 pectoral fin (p-f), which is seen curling round the depressed cheek. Above, the beak 

 is seen to be separated by a deep crescentic sulcus from the rounded cranial sac ; and 



