24 MR. W. K. PARKER ON THE STRUCTURE AND 



Broad in front, and rapidly narrowing backwards, they each have four small tinger- 

 shaped cartilages attached to them, the hindmost of these is confluent with the great 

 plate and the rest are merely articulated with it : these are the rudimentary " cerato- 

 branchials" (c.6r'~^). 



These rays or rudimentary intra-hranchial arches lie in the floor of the throat, above 

 and within the branchial pouches ; but outside and below these there are four pairs of 

 large extra-branch ial cartilages (ex.br^~^.) that form most of the framework of the pouches 

 and lie close within the outer skin — are, indeed, subcutaneous cartilages, like the 

 " labials," and do not belong to the category of true "visceral arches." 



The foremost of these remains free distally, articulating with, the widest part of the 

 hypo-branchial plate ; but the rest have become confluent with it : they all four coalesce 

 with each other, above, like their counterparts in the Lamprey's branchial basket-work. 



The first and fourth of these cartilages are bags or pouches folded into many hills 

 and hollows ; the two in the middle are widish bars. 



The first and second from their hind mai'gin, and the other two from their fore margin, 

 send inward tooth-like rudiments of branchial rays : these agree with the binding parts 

 that make the bars into a basket-work in the Lamprey. 



Li the Sharks (see " On the Selachian Skulls," T. Z. S., vol. x., j^late 38, figs. 1 and 2 ; 

 and (tEgenbaur's 'Selachians,' plates 11-20) we see that the extra-branchials are 

 feeble, the intra-branchials tijpical, and the branchial rays distinct ; in the Skates or 

 Rays these frmging cartilages are much dilated externally, and these dilatations are 

 articulated together and suppress the extra-branchials. 



The two upper labials (Plate 3, figs. 1-3, u.l.) are but little united at the middle, 

 and each has a fenestra in it ; on the right side there is a lesser hole. 



The labials of the sucking disk (I. I.) are larger than in the last instance, and this 

 corresponds with the fact that this species is very generalised and archaic. 



The spiracular cartilage (Plate 3, figs. 1, 2, sp.c.) is confluent with both the " tegmen 

 tympani " and the " otic process " of the suspensorium. 



When these parts are studied, not only after dissection but also with all their 

 enclothing structures, we find that the ordinary Batrachian larva is indeed a kind 

 of " Cyclostomous " fish, and the Lamprey and this larva mutually explain each other. 



Palatal and lingual aspects and a longitudinally vertical section of the head of the 

 largest of these Tadpoles (5 inches long) show many instructive things.'''' 



In the first view (Plate 1, fig. 3), the inferior ai'ches have been removed bodily, and 

 the palate tui'ned upwards ; we thus get a view of the horny jaw-plates, the oral 

 papilla), the quadrate hinge, the condyle for the hyoid, the inner nai'es, and the whole 

 of the palate up to the faucial region. 



The upper part of this Pttrorayzine mouth shows not only the jn-incipal crescentic 



* Profrssor HuxLET (on " Petromyzon,'" ,Tour. of Aiiat. and Pli.ys., vol. x., plates 17, 18, pp. 112-420) 

 Las served as "pioneer" in this part of the tliickot; and a cleared space, sucli as he has made, makes my 

 ■work comparatively easy. 



