DEVELOPMENT OF THE SKULL TN THE BATRACHIA. IG9 



Second suh-division. — Tree-frogs, inth ddated sacred apophyses, and without parotoids. 



Family "Hyud.k." 



First genus. HijJa. 

 45. Ilijla Ew'tngii — Adult female; 1^ incli long. Van Diemen'a Land. 



The lesser, glandless, flat-backed Tree-frogs form a very natural group ; those of 

 the genus Hyla a very neat group. Tlie Australian Anura of various genera and 

 famUies are very frail and delicate in their build, and their skulls especially are often 

 extremely deficient even in cartilage, -uhich, like the outer bone, is often used with 

 the utmost economy. 



This in the case of the HyJa' 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 retiu'n 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 hah" oval ; the bi-eadth is to 

 the length as 9 to 8 ; the occipital condyles (pc.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 tlie fore edge of the stapes ; this is a correlate of the an-ested 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 mto 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 pf the tegmen tympani, and the rim of the 

 fenestra ovahs, is one continuous (generalised) osseovis tract, which reaches up to the 

 optic fenestra (IT.) 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 surfiice. 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 auditoiy 

 capsules are relatively larger, and the parotics less extended, than in the Pelodryadidse. 

 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 tlae 

 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 gudle-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 

 ethmo-palatine bar is segmented in B>ifn.' Here, as in I'elodryas (Plate 34), the na.sal 



MDCCCLXXXI. Z 



