ON THE VERTEBRATE SKELETON. 197 
to prevail, as in the case of the alisphenoid, in the determination of its special 
homology. 
The supraoccipital, by virtue of its internal position and lodgment of part 
of the labyrinth, has equal claims to the name of ‘rocher,’ according to the 
Cuvierian characters of that bone, and Geoffroy St. Hilaire did not make a 
less arbitrary choice in singling out this element as ‘le seul rupéal*,’ than 
Cuvier did in choosing the alisphenoid, or, as any other anatomist would do 
in preferring any other element of a cranial vertebra in the crocodile to 
represent the ossified ear-capsule of the fish or mammal, because portions of 
that ossified capsule are protected by, or have coalesced with, such vertebral 
elements. Had Cuvier looked beyond the special homology of the bones of 
the head of the crocodile, and permitted himself to appreciate their higher and 
more general relations, he could scarcely have failed to perceive the corre- 
spondence of his so-called ‘ rocher’ in batrachians, ophidians, chelonians and 
saurians, to the bone which he so well recognizes as ‘the great wing of the 
sphenoid’ in the perch and cod-fish. 
The Mastoid—tIn the human embryo of the fifth month a centre of ossi- 
fication is established on the outer surface of the mass of cartilage occu- 
pying the interspace between the basioccipital (fig. 11, 1) and exoccipital 
(2) below, the tympanic (2s) and squamosal (27) in front, the supraoccipital 
(3) behind, and the parietal (7) above: this mass of cartilage incloses the 
membranous labyrinth, about which a light osseous crust has begun to be 
formed ; and, from the centre (s) established near the outer border of the 
posterior semicircular canal, ossification radiates to complete that part of the 
cranial parietes, which, in the adult skull, is impressed on its inner surface by 
the great venous channel called ‘fossa sigmoidea,’ and developes from its 
outer surface the ‘ processus mastoi- 
deus.’ The primitive independence 
of the base of this process, which 
Kerkringius so clearly and accurately 
delineates in his tab. xxxv. fig. iii. as 
the posterior of his ‘tria petrosi ossis . 
distincta ossicula+,’ is a fact of much 
more significance than its brief and 
transitory manifestation would lead 
the anthropotomist to divine. The 
coalescence of the primitively distinct 
mastoid with the ossifying capsule of 
the labyrinth is very speedy, being 
usually complete before the foetus has 
passed its fifth month, and a com- - 
posite ‘ petro-mastoid’ bone is thus 
formed, which, retaining its indivi- 
duality in monotremes, marsupials, 
ruminants and many rodents, pro- Skull of the human embryo ; fifth month. 
ceeds to coalesce with the additional as et 
elements of the ‘temporal’ bone in man, and with other surrounding cranial 
bones in birds. In the cold-blooded vertebrata, the mastoid retains, with a few 
exceptions, its primary embryonic distinctness, as an independent element of 
_ the skull. In tracing the modifications of this element downwards from man, 
we find the external process from which its anthropotomical name originated, 
* Annales des Sciences Naturelles, tom. iii. 1824, p. 271, pl. 16. 
T Spicilegium Anatomicum, 4to. 1670, Osteogenia Foetuum, p. 269. 
