1864.| Comparative Anatomy and Classification. 543 
mere centrum, and its correlation with the nervous axis completely 
lost. 
Mr. Huxley has devoted much care and labour to the determi- 
nation, throughout the vertebrate scrics, of the bones which are homo- 
logous to the petromastoid portion of the human temporal bone, and 
the mode of their development. Partly by a critical inquiry into the 
almost forgotten researches of Kerckringius and Cassebohn, and 
partly by the more recent observations of Meckel, Hallmann, and him- 
self, he has shown that it ossifies from three distinct centres, to which 
he has given the convenient terms of pro-otic, opisthotic, and epiotic 
bones. These bones enclose the organ of hearing and “are very 
generally represented, sometimes in a distinct form, and sometimes 
coalesced with one another, or with other bones, throughout the series 
of skulls provided with cartilage bones ; and the pro-otic especially is 
one of the most constant and easily identifiable bones throughout the 
series of vertebrate skulls.” 
He does not allocate these bones, either in their separate or con~ 
junct capacity, to any of the cranial segments, but regards them, lke 
the osseous chambers of the olfactory organs, as bony capsules inter- 
posed between the arches of the segments. Their true morphological 
position may, however, be still held to be an open question. For 
they are developed in cartilage which forms a fundamental part of the 
primordial cranium, and, as such, it may be and has been argued, both 
by Carus and Goodsir, that they should have a place amongst the 
cranial segments. 
In studying the morphological relations of the inferior, or, as they 
are sometimes called, hemal arches of the cranium, it is of great im- 
portance that the nasal cavities should be carefully examined and the 
position of the nostrils determined. Great weight was attached to 
these points by Mr. Goodsir, in one of the memoirs already referred 
to, and we find that Mr. Huxley has also carefully entered into the 
subject. It has now been satisfactorily determined that the posterior 
nostrils in the mammalia are openings of a totally different character 
from what are called the posterior nares in a bird, an amphibian, a 
snake or a lizard. Inaman, for example, the nostrils open posteriorly 
behind the palate bone, whilst in the other animals named, they open 
in front of those bones, between them and the maxille, and cannot 
therefore be regarded as homologous apertures. 
The nature of the mandibular and hyoidean arches, situated behind 
the orifice of the mouth, has always been a difficult problem for the 
morphological anatomist. The embryological researches of Rathke 
and Reichert have done much to clear up many of the obscure and 
complex questions involved in their investigation. Many sound mor- 
phological data were also furnished by Mr. Goodsir, both as to their 
relation to the cranial segments, and the homology of their constituent 
elements. With much that Mr. Huxley has written we are disposed 
to coincide, though we confess ourselves unable to accept all his pro- 
positions regarding these arches in the present somewhat uncertain 
state of our knowledge of their mode of development. 
