DEVELOPMENT OF THE SKULL IN THE BATRACHIA. 
1G9 
Second sub-division. — Tree-frogs, with dilated sacral apophyses, and without parotoids. 
Family “ Hylidjs.” 
First genus. Hyla. 
45. Hyla Ewingii. —Adult female; 1|- inch long. Van Diemen’s Land. 
The lesser, glandless, flat-hacked Tree-frogs form a very natural group ; those of 
the genus Hyla a very neat group. The Australian Anura of various genera and 
families are very frail and delicate in their build, and their skulls especially are often 
extremely deficient even in cartilage, which, like the outer bone, is often used with 
the utmost economy. 
This in the case of the Hylce is in perfect harmony with the life led by these insect¬ 
like Batrachians, and is very instructive as throwing light upon the influence of 
external conditions upon a most sensitively modifiable group. I shall return to this 
subject again when summarising the whole of this piece of work. 
The skull of this species (Plate 31, figs. 1, 2) is an even half oval; the breadth is to 
the length as 9 to 8 ; the occipital condyles (oc.c.) project but little, are postero- 
inferior, of medium size, and wide apart. The condyles of the quadrate (q.c.) reach as 
far back as the fore edge of the stapes ; this is a correlate of the arrested size of this 
type. The skull is very flat and wide, and very open ; there is one large oval fon- 
tanelle, which reaches almost from the closing in of the cranial cavity in front, to 
some distance into the inter-auditory region, behind ; at the sides, also, the tegmen 
is scarcely developed at all, and thus three-fourths of the roof is membranous. The 
whole occipital arch, and all but the edge of the tegmen tympani, and the rim of the 
fenestra ovalis, is one continuous (generalised) osseous tract, which reaches up to the 
optic fenestra (II.) in front. The flatness of this little skull is such as to throw the 
nerve-passages both before and behind the ears (fig. 2, II., V., IX., X.) on to the general 
plane of the gently convex lower surface. Thus this skull resembles that of a young 
typical Frog, artificially compressed ; and that in spite of the intense ossification of both 
the fore and hind skull. In accordance with the general arrest as to size, the auditory 
capsules are relatively larger, and the parotics less extended, than in the Peloclryadidse. 
So also the mid skull is wider; the cranial cavity is larger, in proportion to the part in 
front, and is twice as long as the nasal region. There is very little pinching in of the 
orbital region, which is widest behind, where the temporal region begins, and in front 
where a distinct eave of cartilage is left unossified ( s.ob .). The girdle-bone (eth.) runs 
into the nasal region in front, transversely above, and as a spike below ; it takes in also 
part of the proper territory of the anterior sphenoid, for more than half of the orbital 
region is ossified by it, and it also runs well into its own wings, stopping where the 
etlimo-palatine bar is segmented in Bufo. Here, as in Pelodryas (Plate 34), the nasal 
MDCCCLXXXI. 
Z 
