DEVELOPMENT OP THE SKULL IN THE BATRACHIA. 
27 
“ The recesses 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 oft 1 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 produced 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 
continued into a more delicate fold of the mucous membrane lining the roof of the 
mouth, the free edge of which also projects backwards. It is plain 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 in tra-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, surmounted 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 Axis”) the gill-folds are formed in a double series of pouches, 
the common framework of which is formed of typical Aim-branchial arches, each 
composed of nine pieces, on the inner side; the septa are strengthened by free 
“ branchial rays,” and on the outside (in the Shark) there are distinct “ extra- 
branchials,” one to each pouch, which are 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 false “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, hr 1 ., fir 2 .); these are developed from the epidermis—are epi- 
blastic. But the tufts that break forth from the clefts in the more developed Tadpole, 
* I have recently Lad my mind set at rest npon tLis subject by my talented friend. 
E 2 
