DEVELOPMENT OE THE SIvHLL IN THE BATRACHIA. 
627 
and cornua. The hyoid is also longer, slenderer, and more loosely attached to the bar 
in front of it (Huxley, op. cit. fig. 2, Hy). 
Now it may be safely taken for granted that the earliest chondrocranium of Dacty- 
letlira would exhibit all the essential Batrachian characters ; and the condition here to 
be described in the earliest stage in my possession agrees with this assumption. 
I shall therefore take the diagrammatic model or platform of the earliest chondro- 
cranium of the Common Toad as the pattern of the foundation of this advanced chon- 
drocranium of Dactylethra (see Plate 55. fig. 1). 
For immediate comparison of a like stage to this, the reader is referred to the third 
stage of the Common Toad’s Skull (Plate 55. fig. 3 ) ; the counterpart of this in the 
Common Frog is seen in Professor Huxley’s last quoted paper (plate xxxi. fig. 3) ; the 
figures given in my former paper (“ Frog’s Skull,” plate v.) correspond very exactly to 
the second stage of Dactylethra (Plate 57. figs. 1, 2). We shall need all these figures, 
and many more, for comparison ; and when this most remarkable, flat chondrocranium is 
interpreted, it will serve as a key to unlock the mysteries of other remarkable chondro- 
crania, both of those which now exist, such as we see in Chimcera and the “ Dipnoi,” 
and of those that did exist , such as Coccosteus and Pterichthys* . 
Surely, in these latter, under the dermal scutes that are preserved there was a chon- 
drocranium — the true endoskeletal skull — whose shape and whose morphological deve- 
lopment must have answered, more or less closely, to that now under review, a temporary 
form and structure in this almost extinct kind of Toad. 
The cranial structures in this Tadpole were much more flattened out than a side view 
of the creature itself (fig. 3) would indicate ; for the outer skin is very loose, and the 
subcutaneous stroma is copious and extremely gelatinous. Unlike what we find in larvae 
of the Common Frog and Toad, these Tadpoles are very delicately transparent, although 
richly supplied with brown pigment. 
In this stage the occipital arch (Plate 56. fig. 4, e.o .) is imperfect above in the hind skull, 
and in front the internasal plate ( i.n.l .) is flat, or rather subconcave, for the ethmoidal wall 
(see “ Frog’s Skull,” plate v. eth.) has not yet been built. At present, then, we have a 
common cranio-nasal valley, whose bottom is formed behind of the parachordal cartilage 
and notochord (iv., nc .), and in front, for twice the extent, of the trabeculae, to whose 
extended edge in the frontal wall a long labial has been attached. 
Laterally, the union of the parachordal bands with the auditory capsules (iv., au .) has 
taken place very early ; and with the antero-internal angle of each of these the trabeculae 
are united, as well as with both the parachordals. The sides of the cranial valley are 
soon embanked into a narrow space by convergence of the trabecular elevations. There, 
at the narrowest, the brain ends, and the. olfactory crura (1, indicated by a dotted line) 
are as long as the brain. Outside the wide precerebral valley we see the mandibular 
arch (pd., q.), to which is attached the hyoid (c.hy.), and outside the mandibular pedicle 
* Note in Hush Miller’s ‘ Old Red Sandstone,’ plates i., ii., iii. & xi. pp. 72, 76, 78, & 279 ; and Huxley, 
‘ Memoirs of the Geological Survey,’ 10th Decade, 1861, p. 30, fig. xix. 
4 s 2 
