OF THE SKULL IN THE UEODELOUS AMPHIBIA. 
563 
was made behind the confluence of the ascending process with the alisphenoidal wall ; 
consequently it also is free in this section. 
This section also shows how the squamosal ( sq .) binds on the outer face of the suspen- 
sorium, and the pterygoid bone (pg.) on its under face. The substance of the suspen- 
sorium is being converted into the quadrate bone (g.). 
One of the most interesting correlations of structure is that of the second basi- 
branchial with the rudimentary larynx in this group. In the perfect Axolotls (Plate 25. 
fig. 5) this part is a slightly gapped, broad-edged chisel ; it is now a delicate fork 
(Plate 27. fig. 5, b.br. 2). More than the hinder half is ossified — all but the tips of the 
prongs, and these have the larynx lying behind and between them. They are related 
to that organ like the single spatulate “urohyal” of a bird*. 
This is only a step, however, to complete separation of the second basibranchial from 
its old attachments (Plate 25. fig. 6, and Plate 27. fig. 7, b.br. 2). 
These modifications, which have taken place in a full-sized Axolotl, whose gills were 
only partially absorbed, are but faint indications of what the innate metamorphic energy 
is capable of performing, when the changes take place timely, so as to convert the 
Axolotl into a kind of Salamander. 
Tenth Stage. Skull of an adult Amblystoma opacum, 3 inches 10 lines long. 
This is a North-American species, closely related to, if not identical with, the form 
into which the Axolotl changes when its metamorphosis is complete. 
The whole appearance of the skull is changed ; it is altogether neater, more solid, 
and its narrowed, more elongate form is much more like that of a Frog. 
In fact it has undergone changes quite similar to those which the skull of a meta- 
morphosing Frog undergoes in passing from the short-tailed condition to that of the 
adult. 
Nevertheless the adult skull of the Caducibranchiate Urodeles and that of the 
Batrachia are very different ; the former types belong to a lower and more lateral fork 
than the latter, but both are terminal or culminating. There is no inosculation. 
Indeed, whilst the Urodeles may be considered as an upgrowth from the Dipnoi, the 
Batrachia must needs have had ichthyic forefathers, in whom the characters of a Lamprey 
and a Chimeera were combined (“Frog’s Skull,” pp. 193, 194). 
The process of ossification in the occipito-auditory region has been very intense, and 
has obliterated all the sutural landmarks. 
* In one of our discussions Prof. Huxiey controverted my description of the formation of the “ thyro-hyals ” 
of the Frog (see “ Frog’s Skull,” p. 172), where they are described as primarily symmetrical and double, because 
his observations on the Newt showed that its thyro-hyal is formed of a basal azygous piece. They are double, 
however (as in Mammals), in the Batrachia. That instance, of inferring from the Urodele what would he found 
in the Batrachian, was parallelled by me when I inferred from the larval Salamander that the stapes of the Frog 
would he developed by segmentation off of a part of ready-made cartilage taken from the ear-capsule. 
4 k 2 
