DEVELOPMENT OE THE SKULL OF THE COMMON FROG-. 
197 
it will be seen that, whilst the first and second postorals are similarly distinct, yet there 
is a great difference between the Fish and the Frog. In the Fish the second postoral 
not only diverges , it also bifurcates, an anterior slender fork passing down close behind 
the preceding bar in front of the gaping space (Sg.), and a posterior stout fork passes behind 
this space the slender “symplectic” fork ossifies separately (Elem. fig. 72, B, C, 
Sg.) ; another bony sheath encloses the broad-topped upper piece, “ hyo-mandibular” 
( II. M .) ; the top of the diverging bar becomes segmented off and ossified as the “ stylo- 
hyal and the remainder becomes a thick rib of bone, from which a distal segment is 
cut clean off as the “ hypohyal.” Both this latter and the larger piece are ossified by 
two centres each. Above the process which becomes the “ stylo-hyal” there is a knob, 
the “ opercular process,” the part which becomes the “ extrastapedial” of the Frog. 
The synchondrosis between the “ hyo-mandibular” and the symplectic of the Teleostean 
Fish does not correspond with the joint-cavity which passes through the “hyo-mandi- 
bular” itself in the Sturgeon and the Frog. 
The symplectic is a mere rudiment in my third stage of the Frog, but it developes 
more afterwards ; it is persistently free from the quadrate in most Teleosteans, but early 
coalesces with that part in the Eels (e. g. Anguilla acutirostris ): in these Fishes the pterygo- 
palatine connective is early aborted, and is feebly indicated in very young individuals 
by a delicate rod of ossified membrane, the “ pterygo-palatine ” bone. 
C. Comparison of the Frog's Skull with that of the Urodelous Amphibia. 
These lower forms of Amphibia lie between Lepidosiren and the “ Anura.” They 
agree largely with the former ; but the pterygo-palatine cartilage is very much aborted, 
as in the Eel. There is a “ stapes but there is no metamorphosis of the top of the 
hyoidean bar to form any secondary elements to the “middle ear.” 
D. Comparison of the Frog's Skull and Face with that of the “ Sauropsida." 
1. With the Reptilia. — This comparison has been in some degree anticipated by my 
use of Professor FIuxley’s terms, as given in his paper on the prototypes of the Malleus 
and Incus (Zool. Proc. 1869). 
My first and second stages throw an unexpected light on the hyo-stapedial structures 
of Sphenodon {op. cit. p. 397, fig. 4). The second postoral of this Lizard has coalesced and 
retained its coalescence with the auditory mass. It has not segmented itself into upper, 
middle, and lower parts, but the “supra-” and “ infrahyomandibular” regions are per- 
manently continuous with each other and with the “ stylo-cerato-hyal.” The semicir- 
cular scooped “extrastapedial” ( E.St ) is the “opercular process;” and from this there 
grows backwards a “ medio-stapedial” bar, which is continuous with the cartilage that 
is ossified to form the columelliform rod of the “ stapes here the stapedial plate 
early formed a union, by means of a secondary connective bar, with the continuous (unseg- 
mented) hyoid arch. 
W ith regard to the “ pterygo-palatine connective,” it may be remarked that the Rep- 
tilia develope but little cartilage, and that of a very simple type, in this region ; it is 
