DEVELOPMENT OE THE SKULL OF THE COMMON FROG. 
193 
forms a perfect ring in the Bull-frog, its ends becoming completely confluent; its 
“ pterotic ” and “ epiotic ” regions, although soft, are as well developed as in the “ Tele- 
ostean” Fishes. 
One thing appears to be quite unique, although it will perhaps turn up in some other 
type, and perchance in the extinct “ Labyrinthodont this is the presence of an ante- 
rior “ parasphenoid,” the fore part of the “ rostrum ” being separately ossified. 
A similar breaking-up of centres is seen in the palatines, maxillaries, “ infrahyoman- 
dibulars,” &c. 
My great use of the larval Pseudis has been to obtain a thoroughly satisfactory eluci- 
dation of the formation of the nasal alse and septum ; and in it I first clearly saw that 
the pterygo-palatine band was an entirely secondary growth : in this type there is another 
“ connective ” in front of the nasal opening, from the inner angle of the quadrate to the 
trabecular horn. In Bufo vulgaris the “ extrastapedial ” is shaped like a peltate leaf; 
its “ suprastapedial ” is small, terete, and free ; its “ medio-stapedial” bar is very long, 
and is ossified by a small shaft bone where it passes into the “ extrastapedial,” and the 
rest by a larger shaft. 
The “ stylo-cerato-hyal” is very large in the larval condition; it undergoes the same 
morphological changes as in the Frog, and ultimately coalesces with the auditory sac, 
as in the Mammalia. 
B. Comparison of the Frog's Skull with that of various ichthyic types. 
1. With Petromyzon marinus. — In the larval stage (“ Ammoccetes ”) as given by 
Muller {op. cit. pi. 4. figs. 6-10), the “trabeculae” have coalesced, but the “horns” 
have not budded out ; thus it is later than my second stage, and earlier than the third. 
The author has not shown the other facial bars ; the first pair (D), as here figured, have 
already become nearly straight, and, besides uniting by the “ anterior commissure,” they 
have coalesced with the “ investing mass ” (d). 
In the adult (op. cit. figs. 2-4) an amount of morphological change has taken place 
wholly beyond what might have been expected in so low a Fish. „ The three foremost 
pairs of arches have coalesced with each other above, and also with the investing mass. 
In front of the “ commissure ” the trabeculae have developed an azygous bilobate plate, 
the equivalent of the symmetrical “ horns ” of the larval Frog ; and their immediate suc- 
cessors, the mandibular bars, have each sent forwards a styloid rudiment of Meckel’s 
cartilage (fig. 2, i), which, however, like its trabecular counterpart, is not segmented off'. 
The pterygo-palatine bar (7) is as large as in my “ fifth stage.” The hyo-mandibular (i 1 ) 
diverges early from the mandibular pier, with which it is largely confluent above ; it has 
cut off a “ stylo-cerato-hyal ” (i"). 
The branchial arches (d,d,d) are tied together above by a continuous “ connective,” 
which runs into the fused roots of the mandibular and hyoid arches. They are converted 
into an exquisite piece of basket-work by being extensively bound together by similar 
secondary growths. (Owen, 4 Lect. Comp. Anat.’ p. 52, fig. 11.) 
MDCCCLXXI. 2 D 
