274 MR. W. K. PARKER ON THE OSTEOLOGY OF BALANICEPS REX. 
reptiles, the central is the chief element. In Coluber natriz and in Lacerta agilis, Rathke 
has observed the epiotic centres before they had coalesced with the main part of the 
supra-occipital, and the mastoid element before it was fused to the ex-occipital (see 
Huxley, Croon. Lect. p.61). In most osseous fishes the three supra-occipital elements 
continue distinct throughout life. 
According to Professor Goodsir, the interparietal of the mammals is part of another 
sclerotome, being the homologue of the divided roof-bone of the temporal cincture, the 
so-called ‘ parietals’ of oviparous vertebrata: Professor Goodsir calls these upper tem- 
poral elements temporo-parietals. 
The mastoid would appear to be present in Colymbus cristatus (in very young birds), 
from some observations made by Dr. Hallman (Croon. Lect. p. 54); but examination 
of the development of Divers, Auks, Penguins, and allied birds is very desirable ; and 
perhaps the Struthionidz, taken early enough, might reveal a distinct osseous centre for 
this bone. 
We consider Cuvier’s ‘ rocher’ in fishes and Professor Owen’s ‘ petrosal’ to be the 
mastoid ; the large bone notched or perforated in front by the trigeminal nerve being, 
as Professor Huxley has unanswerably shown, the true petrosal (Croon. Lect. p. 24). 
In the Common Mole, Talpa europea, the mastoid is very large, and forms more of 
the cranial wall than its serial homologue, the squamosal ; in this creature the petrosal 
forms nearly the whole of the labyrinth. 
That the mastoid is not necessarily connected with the petrosal is shown in the skull 
of the Common Bat, Vespertilio murinus ; for in this instance (and apparently in the 
Brazilian, Molossus obscurus), the mastoid is anchylosed to the ex- or lateral-occipital, 
and is only connected by membrane to the squamosal and petrous bone, the latter 
bone being totally distinct from all the surrounding elements. 
We now return to the occipital bone of adult birds. 
In the Hornbills (Buceros), where the mere size of the head (although extremely light) 
causes the expenditure of much muscular force, a tubercle exists above the foramen 
magnum, for the attachment of a very thick elastic ligament: this tubercle is wanting 
in the Baleniceps and the Adjutant. 
In the Common Heron the occipital resembles that of the Balzeniceps, but it is higher 
in proportion to its width; its upper boundary is almost straight, there being only a 
slight angle in the centre, where the transverse occipital crest is confluent with the 
sagittal ; the par-occipital processes also are relatively much smaller. But the elegant 
basi-cranial pterapophyses, and the open Eustachian groove between the proper basi- 
sphenoid and the basi-temporal, are singularly like what we find in the Balzniceps. 
The Adjutant and the Albatros have this canal open in the dry skull, but in most 
birds it is more or less covered in, leaving only a central aperture. 
In the Pelican the line of junction between the occiput and the upper part of the 
skull forms a very obtuse angle, these surfaces being at nearly a right angle in the 
