DEVELOrMENT OF THE SKULL IN THE BATRACHIA. »1 



ear-shaped flap, notched below, and in front sending down a membranous tract, wliich 

 will soon become cartilage. 



The broad part of this rudimentaiy " annulus " lies on the swollen unossiiied 

 tegmen tympani, covering the ampulla of the horizontal canal (figs. G, 7, h.s.c), and 

 forming a bridge over the 7th nerve and the tympano-Eustachian cleft, which opens 

 inside, between the suspensorium and the condyle of the pedicle (Plate 12, fig. 3). 



In the recess beneath this double roof the epi-hyal element (columella) has now 

 become almost normal in form, but is not yet ossified (figs. 4, G, 7, co.). 



Although relatively so much smaller, it is no hard task to harmonise this element 

 with the hyo-mandibular of a Skate, which is developed quite independently of the 

 cei-ato-hyal, and perchance a little after it in point of time. 



There is but one piece of cartilage, for the inter-stajiedial segment is not distinct ; 

 this jDart is bi'oad, oblique, and wedges in between the ear-sac and the stapes (st.), now 

 a large lozenge-shaped cartilage. 



The extra-stapedial region is bent downwards, it is ligulate, and retains the breadth 

 of the medio-stapedial region ; there is no supra-stapedial process. 



The mandible has more than doubled the length It hnd in the last stage (Plate 12, 

 fig. 1, mk.) ; the angular process is reduced greatly, uik; ihe loosely swinging hinge 

 is quite unlike that of the larva. 



The inferior labial has coalesced with the distal end of Meckel's cartilage [mk.), 

 but the dentary (d.) has not yet converted it into a " mento-Meckelian " bone. The 

 proximal broad end of the lower jaw lies in a thin bony trough, the "articulare" (ar.); 

 this is developed most on tlie inside. 



The cerato-hyal {c.hij.) is now attached by the " hyo-suspensorial ligament " (fig. 4, 

 h.s.l.) to the inside of the suspensorium at its lower third ; its styloid region (fig. 4, st.h. ) is 

 somewhat uncmate, and its hypo-hyal region {h.hy.) decurved ; the basi-hyal has vanished. 



A single rudiment (fig. 5, ex.hr.) on each side remains of the four inner and foiu' 

 outer branchials ; this plate embraces the basi-branchial (b.br.) which is wider than long, 

 and has a crescentic emargi nation in front. 



The pair of hypo-branchials (h.hr.) are not much altered, but their hinder part, which 

 sends out rudiments of a third and foin'th cerato-branchial (Plate 11, fig. 4, c.hr., 3, 4), 

 has now become a solid *' horn," ready to ossify as a " thyro-hyal " {t.hy.). 



This is a very instructive stage of the hyo-branchial apparatus ; if it be compared 

 with the larval structures (Plate 10, fig. 6 ; and Plate 11, tig. 4), and with those of an 

 adult Frog (see in Rana pipiens, Plate 8, fig. 5), we shall see what becomes of the 

 copious cartilaginous growths of the larval branchial arches. 



Before leaving this stage I have to remark upon the e^^dence these changing larval 

 skulls supply of the existence of a pre-oral visceral arch in the ethmoidal region. 



Years ago my study of the development of the skuU iu Birds and osseous Fishes, 

 besides what I saw in various kinds of adult Birds, left me iu a state of mi) id that 

 scarcely admitted a doubt upon the suljject. 



MDCCCLXXXI. M 



