OF THE VOMEE, ETHMOID, AHD INTEEMAXILLAEY BONES. 
317 
the orbits in Whales. In other animals the arrangsment is perhaps not so distinct, 
and there may be a difficulty, especially in mammals, to determine if the nasals do not, 
in part, at least, form a continuation of the roof of the cranial cavity ; for in them the 
neural arch of the ethmoidal sclerotome is, as we have seen, a continuation of that 
ca'vity, and is open in front. But, inasmuch as in the mammal the olfactory-sense cap- 
sules are withdrawn into the interior of the ethmoidal neural arch, the cranial cavity is 
closed in by soft parts at a point posterior to the nasals ; and I therefore incline to think 
that the nasals, protecting in this, as in other cases, the nostrils, are, like those of the 
Frog, similar in nature to the orbital processes of the cetacean frontals. 
Note. — Since writing the above, I have deemed that it would be advisable to add here 
some further details as to the morphological structure of the cranium in the mammal, 
even though it is impossible in this place to give the arguments on which these details 
are founded. The descending process of the occipital bone, the mastoid, the squamous, 
the external angular process of the frontal, and the lacrymal are serially corresponding 
structures, belonging respectively to the occipital, petrous, parietal, frontal, and ethmo- 
vomerine segments, and tending to project from their neural arches. There are no 
haemal arches in connexion with the protective segments. Into the composition of the 
haemal arch of the petrous segment enter the styloid processes, the stylohyoid liga- 
ments, and the small cornua of the hyoid ; while the great cornua of the hyoid belong 
not only to a segment behind, but to the true splanchnic skeleton ; that is to say, to 
the series of sclerous structures internal to the primary vascular arches, and which are 
best developed in the branchial arches of fishes. The other haemal arches are, as has 
been stated, the pterygoid, palatal, and maxillary, belonging respectively to the pre- 
sphenoid, ethmovomerine, and facial segments. The maxillary arch, having assumed in 
the mammal as much as possible the aspect of a neural arch, as has been above explained, 
although in reality haemal, has a bone radiating from it — the malar, which tends to form 
connexions with the bones radiating from the neural arches behind, viz. with the lacry- 
mal, external angular process of the frontals, and the squamous. The lower jaw is not 
a haemal arch at all ; that is to say, it is not an arch belonging to a single segment and 
corresponding to the palate-bones, or to a pair of ribs, but is a limb-arch. For I hold 
that Professor Goonsm has distinctly shown that the shoulder-girdle and pelvic girdle 
are not rib-arches, and that limbs do not belong merely to single segments ; but seeing, 
on the other hand, that these girdles pursue an arched direction, I conceive that they 
are to be considered as arches external to the series of rib-arches, and supporting radia- 
tions, just as rib-arches sometimes do. Such a limb-arch is the lower jaw ; but the 
visceral laminae being very imperfect in the head, it is intimately connected with arches 
bounding the haemal cavity. In the fish it supports extensive radiations, viz. the oper- 
cular apparatus, but in the mammal it bears no radiations. The quadrate jugal in the 
bird has every appearance of being a radiation homologous to the operculum ; for not 
