206 REPORT—1846. 
dences of the homology of the mastoid is shown by the second synonym, ‘ os 
petrosum,’ which it has received from the justly-celebrated author of this 
instructive memoir (pl. 20. figg. 10, 12, 13, 14, p). The actual capsule of 
the membranous labyrinth is covered by the mastoid and exoccipital, and 
remains wholly cartilaginous, as in other ophidia ; and as it likewise does in 
Rhinophis, where its name ‘ petrosum’ is in like manner transferred by Prof. 
Muller to the coalesced mastoid and alisphenoid. In Cheirotes the course 
of confluence proceeds to obliterate not only the suture between the mastoid 
and alisphenoid, but that between the mastoid and parietal; as also of those 
between the frontal, parietal and supra-occipital; the whole cranium pre- 
senting almost the extent of coalescence which characterizes the hot-blooded 
bird. Only the immediate covering of the membranous labyrinth remains 
cartilaginous. 
The sides of the superior surface of the cranium of bony fishes usually 
extend outwards as a strong irregular ridge, from which three processes more 
particularly project, which are supported by three distinct bones, suturally 
united, and each impressed with an articular glenoid cavity. And here I 
cannot avoid remarking how beautifully the principle of vegetative repe- 
tition* is exemplified in the lowest class of the Ve: iebrata, where conse- 
quently the relations of serial homology of the parapophyses in question are 
unmistakeable. The posterior process or bone which sustains (in part) the 
scapular arch is the paroccipital (fig. 5,4); the anterior one, which sustains 
in part the tympano-mandibular arch, is the post-frontal (2b. 12); and the 
intermediate and usually most prominent bone (75.8), which sustains in part 
the epitympanic (28a), and through that the hyoid arch, is the homologue of 
the bone whose essential characters have been discussed under the name of 
‘mastoid.’ The paroccipital having now risen to a level with the mastoid, 
this forms the second strong transverse process at each side of the cranium. 
The process is developed from the outer margin of the mastoid; the inner 
side of the bone is expanded, and enters slightly into the formation of the 
walls of the cranial or rather the otocranial cavity, its inner, usually cartila- 
ginous surface lodging the fibro-cartilaginous continuation of the petrosal 
which immediately covers the external semicircular canal. It is wedged into 
the interspace of the ex- and par-occipitals, the petrosal, the alisphenoid, the 
parietal and post-frontal bones.. The projecting process lodges above the 
chief mucous canal of the head, and below affords attachment to the epi- 
tympanic or upper piece of the bony pedicle from which the mandibular, 
hyoid, and opercular bones are suspended : its extremity gives attachment to 
the strong tendon of the dorso-lateral muscles of the trunk. 
It might have been supposed that this contribution to the walls of the 
cranial cavity, ‘this articulation to the occipital and tympanic bones, all of 
which are constant characters of the mastoid in mammals, and but occasional 
ones in the squamosal—not to speak of the apophysial form and functions of 
the bone in question in the skull of fishes—would have made the balance in- 
cline to the choice of the ‘ mastoid’ rather than of the ‘squamosal’ elements 
of the human temporal in the judgement of every unbiassed investigator of 
its homologies. The German anatomists, however, in falling with Cuvier 
into the mistake respecting the homology of the ‘ mastoid’ (no.s) in birds, 
with the squamosal in mammals, adhere more consistently to their error and 
continue to apply the name ‘squamosal’ or its equivalents to the homologous 
bone in reptiles (fig. 22, 8) and fishes (fig. 5, 8). 
* This principle or law is explained in the first volume of my Hunterian Lectures ‘ On the 
Invertebrata,’ in which classes of animals it is necessarily most strikingly and fully exem- 
plified. 
