OF THE SKULL IN THE UEODELOUS AMPHIBIA. 
555 
has become fused with the flat part of the trabecula, the common basal plate so formed 
plasters, as it were, the antero-internal face of the ear-capsule. In that type the prootic 
appears early ; but when largely spread over the sac, this bone has, covering its fore 
part, a mass of cartilage derived from the basal plate. The socket for the pedicle is 
sunk in the lower part of this investing cartilage, and, above, it appears as an outstanding 
wing over the suspensorium and between the ascending and otic processes*. The 
seventh nerve (7 2 , Plate 24. figs. 2 & 4) escapes from the skull directly beneath this 
embracing cartilage ; the tract in front of that nerve belongs to it, and not to the 
auditory sac. 
In this stage the prootic has not yet made its appearance ; nor is it seen in much 
larger specimens. 
The osteocranial elements are just becoming perfect ; the squamosals especially (sq.) 
are broadening, and show now that peculiarity of their form in the Axolotl’s skull, 
namely, the fingered splinters at their upper or auditory end. 
But the most important thing to be noticed is the segmentation of the small denti- 
gerous palatine from the large pterygoid wing which it had acquired in the fourth stage 
(Plate 22. figs. 4, 5, p.pg-). 
Both these newly disparted bones are blunt-pointed where they overlap, having been 
parted asunder obliquely. 
I have long been familiar with the segmentation of the bony bar which in the embryo 
of Carinate birds runs from the quadrate to the vomer. There , however, it is the 
mesopterygoid which is cut off from the front of the pterygoid, to be added to the 
palatine by ankylosis. Here the primary palatine acquires a pterygoid and then moults 
it again, and the two bones diverge (in the highest or Salam android stage) from each 
other ; this will be explained in the tenth stage. 
The parasphenoid (Plate 24. figs. 2 & 4, pa.s .) is now a very elegant and delicate 
lamina of bone, almost oblong ; it has a rounded emargination in front, has basitemporal 
angular processes, and is rounded behind, where it only ends with the median part of 
the investing mass ; so that it partly underlies the internasal plate in front, and behind 
reaches the foramen magnum. The rough bony sheath ( o.nc .) of the fore end of the 
notochord lies on this bone, and is early more or less united with it. It is seen that 
both the maxillary and premaxillary form an eave to the adze-shaped trabecular cornu ; 
the former, however, is related also to the nasal capsule and to the outside of the 
ethmoidal region (figs. 1-3). 
Seventh Stage. Young Axolotls 3J inches long. 
This young Axolotl was 1 inch longer than the last ; it shows several things worth 
notice, and amongst them a curious want of symmetry, right and left, the left auditory 
* Prof. Huxley mentions this process in the Siren’s skull (Art. “Amphibia,” Encycl. Brit. p. 7-58). It is the 
“sphenotic” process of the chondrocranium, and is common in Selachian, Ganoid, and Teleostean Fishes. 
Wiedeesheim figures it : see his last work, plate i. figs. 11, 12, V.F. 
4 i 2 
