218 ME. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 



the flattened eye-balls are carefully lodged in sockets, the inner half of which, is formed 

 by the semicircular superorbital cartilages, the horns of which are grafted on to the 

 nasal sac in front and to the auditory sac behind. Outside the posterior horn of the 

 superorbital (sphenotic process) is the first postoral cleft, or spiracle (PI. XL. figs. 2 & 3, 

 cl 1) ; its narrow inferior part has been filled in, and only the upper expanded end has 

 kept open — widely open, however, and showing on the spiracular or metapterygoid car- 

 tilage a comb-like " pseudo-branchia " (ps.br), composed of about eight branchial papillae. 

 Behind the spiracle the gill-bearing hyoid and the proper branchial arches are seen 

 elegantly spreading into a U-shaped system of pouches, growing, as it were from a mas- 

 sive stalk, the occiput and spine. 



Below, behind the basitrabecular beak, the fronto-nasal process (n.f.p) is veiy large 

 and persistent, forming a free, anterior, emarginate lip, with the valvular nasal openings 

 (no) in their primordial ventral position, with enclosed " labials " to guard the openings. 



The angles of the mouth are lipped at right angles to the transverse inferior oral 

 opening ; but the mandible has its gums and teeth quite bare. The angulo-labial fold 

 is continued backwards as part of the general opercular skin, which further backwards 

 and outwards is imperfect in five places on each side. These places are the branchial 

 clefts ; they are the retained lower ends of the huge primary clefts. The general oper- 

 cular fold which has covered the open grating to so large an extent is seen to develop 

 a special ear-shaped flap to each of the branchial outlets. 



Embryos as large as this, when carefully examined, have protruding from their 

 branchial clefts what might, at first sight, be mistaken for a parasitic growth of fila- 

 mentous confervae. These, however, are the still retained external branchiae. They have 

 begun to shorten in the first opening; but most of them are very long (PI. XL. fig. 1, 

 e.br). They are spatulate at their free ends ; and their single vascular loop is still 

 functional. 



Between and behind the branchial apparatus is the large, short umbilicus (u), which 

 connects the embryo with a yolk the size of a dove's egg. 



The chondrocranium, with its appended basketwork of visceral arches, is now com- 

 plete, both as to chondrification and segmentation. 



On the whole very similar to that of the Shark at the same stage (PL XXXVI. 

 figs. 3, 4, 5, and PL XL. figs. 4, 5, 6), it yet difi'ers in several important points. 



The cranium seen from below (PL XL. fig. 6), with the eye-halls removed, is seen to 

 be composed of four pairs of primary elements. Behind, the " parachordal " cartilages 

 that invest the notochord (i v, nc) have coalesced with the hinder pair of " paraneural " 

 capsules, those of the ear. This four-sided shorter hind part of the basis cranii has 

 coalesced in front with another pair of outspread cartilages — the trabeculae {tr) ; and 

 these, also, in front have coalesced with the foremost paraneurals — those of the nose 

 {no). Where the four basal plates meet, the internal carotid arteries [i.c) enter; and 

 in front of these is the appearance of a space and a slit, not so densely chondrified : 



