DEVELOPMENT OF THE SKULL OF THE COMMON FROG. 
195 
of interest are the large size of the pterygo-palatine band, the abortion of the first post- 
stomal arch down to the giving off of the pterygoid (Muller, a ; Huxley, li\ the im- 
mense Meckelian bars, and the equally immense “ hyo-mandibular,” which ends abruptly 
at the point where the lower half of the bar bent backwards, to be segmented off as the 
free hyoid crus (Huxley, fig. 80, g, Hy ). The suspensorial part of the palatine rolls 
freely on the prefrontal part of the trabecula ; and it sends forth a large “ prepalatal” pro- 
jection, like that of the Frog, but much larger. 
In the Skate (Raid) the trabecular horns, after they have coalesced, give off a long 
“prenasal” or snout-cartilage, as in the Bird; in the Sharks, as a rule, there are besides 
this an additional pair growing from the nasal walls, and converging in front to the tip 
of the azygous rod, to form the skeleton of the “ breakwater.” 
The metapterygoid region of the mandibular arch is not always absorbed ; in ~N archie 
(Muller, op. cit . pi. 5. figs. 3, 4) a slender /-shaped free metapterygoid exists above the 
pterygo-quadrate region, and the roof of the palate has in it a pair of free cartilages. 
Here the upper labials are single on each side * above and below. 
My third stage well illustrates the “ placoicl” type of skull, especially if we suppose the 
metapterygoid to be either aborted or segmented off (see Plate IV. fig. 7, and Plate X. 
fig. 13). If this stage be examined, it will be seen that the tops of the trabecular and 
mandibular bars are as yet unconformed to the membranous brain-sac: compare this 
state of things with what is seen in Carcharius (Huxley and Hawkins’s Atlas, plate 5. 
fig. 4), where a bowed band of cartilage grows out behind the optic foramen, and regains 
the skull in front of the hyo-mandibular. Here, I doubt not, the upper free ends of the 
two foremost facial bars had coalesced by a “ connective,” and had passed into the “ in- 
vesting mass” behind ; the trabecular bar then grew inwards to form the subcranial beam, 
whilst a large portion of the second bar became absorbed, leaving only the quadrate 
angle with its huge forthstanding pterygo-palatine in front, and having the equally large 
Meckelian segment articulated to it below. 
4. With the Skull of the Sturgeon ancl Spatularia.' — In Spatularia (Muller, op. 
cit. pi. 5. fig. 7 ; Huxley, Elem. p. 202, fig. 81) the palatine freely moves on the pre- 
frontal region of the highly modified “ trabecula,” and the metapterygoid region is 
wholly absorbed, as in the generality of the Placoids. But the second postoral arch is 
divided in a new manner. The entire bar is divided into three main portions, namely : — 
a “ suprahyomandibular,” which is very large ; then a shorter piece, which is the infra- 
hyomandibular” and “ symplectic” in one ; and then the free “ hyoid cornu” (Muller, op. 
cit. pi. 5. fig. 8, d, f li). The projecting lower end of the “suprahyomandibular” is 
the “ opercular lobe,” and carries the fan-shaped radiating “ opercular.” 
On the whole the Sturgeon agrees with Spatularia; but there are important dif- 
ferences. I cannot find space to describe the immense but quite intelligible modifica- 
tion of the trabecular bands (Muller, op. cit. pi. 9. fig. 10). The pterygo-palatal con- 
* These two pairs of cartilages appear to correspond to the azygous lozenge-shaped piece iu the Sturgeon 
(Mvxleb, pi. 9. figs. 10 & 11 A, b ). 
2 d 2 
