DEVELOPMENT OF THE SKULL IN THE BATRACHLA.. 27 



" The X'ecesses at the sides of the floor of the pharynx into which the interseptal 

 clefts or internal branchial clefts open, answer, taken together, to the branchial canal 

 of the Lamprey, which is not shut off from the oesophagus in the Ammocoete. The 

 anterior boundary of each of these recesses is marked by a fold of the mucous mem- 

 brane, the free edge of which projects backwards, and is prodviced into papilliform 

 angulations so as to appear scalloped. The anterior face is concave. The inner angle 

 of each fold passes into its fellow by a ridge, produced into one or two papillae, which 

 is closely adherent to the median part of the floor of the mouth. The outer angle is 

 contmued into a more delicate fold of the mucoivs membrane lining the roof of the 

 mouth, the free edge of which also projects backwards. It is jilain that these struc- 

 tures answer to the pharyngeal velum of the Lamprey." 



The "inferior velum," or membranous fold (l.v.) will be best understood by reference 

 to both the figures of its natural condition (Plate 1, fig. 4) and also to those of its 

 dissected framework (Plate 2, fig. 8). 



We see at once that this scalloped fold is formed upon the rudimentary arches that 

 grow from the oblique sides of the hypo-branchial plates, and that there is the common 

 pharyngeal covering of these rudimentary iV/fra-branchial arches (" cerato-branchials'"). 



They are able to be the skeleton of a free, scalloped, papillated fold in virtue of 

 their arrested condition ; if they grew upwards round the whole circle of the throat 

 this structure could not exist. 



If each of these rudimentary arches was continued upwards, svu-mounted on each 

 side by an "epi-branchial," and this in turn by a "pharyngo-branchial," we should have 

 such arches as are seen in Selachians, Ganoids, and Teleosteans. 



If the lower, or outer, surface of these arches was beset with a double row of pecti- 

 nated branchial folds, then the Ganoid and Teleostean type of gills would exist; in the 

 Selachians ("branchiis fixLs") the gill-folds are formed in a double sei'ies of pouches, 

 the common framework of which is formed of typical infra-branchial arches, each 

 composed of nine pieces, on the inner side; the septa are strengthened hj fire 

 "branchial rays," and on the outside (in the Shark) there are distinct " extra- 

 branchials," one to each pouch, which aie pointed above and pedate below. 



The "branchial canal" of the Lamprey is correlated with suppression of the 

 " cerato-branchial " rudiments, such as are seen in the Tadpole ; but there is a hyoid 

 arch. 



With respect to the branchial tufts that are so copiously developed within and at 

 the edges of the gill-pouches of the "Phaneroglossa," I find that my own views are in 

 accord with those of Mr. Balfour.^' 



There are true and fahe "external gills": the first, only, are present in the 



"Urodeles," and for a short time in the newly-hatched Tadpole (see Phil. Trans., 1871, 



Plate 3, figs. 2 and 10, br\, hr~.) ; these are developed from the epidermis— are epi- 



hlastic. But the tufts that break forth from the clefts in the more developed Tadpole, 



• I have recently had my mind set at rest upon this subject by my talented friend. 



E 2 



