OF THE VOMEE, ETHMOID, AND INTEEMAXILLAET BONES. 
311 
was the true intermaxillary, were it not that the bones universally acknowledged as inter- 
maxillaries in the fish have claims to the name which cannot be set aside, inasmuch as 
a complete chain of forms of intermaxillaries, of which they form one link, can be traced 
from reptiles to fishes through the Batrachia. The position therefore which I maintain 
is this, that in the sclerotome formed by the nasals, intermaxillaries, and maxillaries, 
when the centrum is represented by a bony formation, it is derived in mammals, birds, 
and reptiles from the intermaxillaries, and in fishes from the nasal : the Ornithorhynchus 
being the only animal, as far as I know, in which it is a separate bone. 
If it be repugnant to any preconceived notion to believe that the same morphological 
element (the centrum) can be derived in one instance from one developmental element, 
and in another instance from a different one in the same segment, we have only to keep 
in memory the exactly analogous instance of the intermaxillaries in the human subject, 
which usually arise from the maxillaries, and yet in certain conditions of development 
take origin in connexion with the septal cartilage (see above, p. 303). 
To return to the series of forms of intermaxillaries in reptiles and fishes : the inter- 
maxillaiies of the Frog are two small bones articulating with one another in the middle 
line, and externally with the anterior extremities of the elongated maxillaries; they 
give off processes towards the nasals, and very slight mesial-palatine processes. Exactly 
in the same way do the intermaxillaries of the Salmon articulate with one another, and 
with the maxillaries ; the only difference being that the maxillaries hang downwards 
instead of being directed horizontally backwards. In most fishes the intermaxillaries 
have superior mesial processes, which pass backwards over the nasal, and evidently 
correspond with the superior mesial processes in frogs, lizards, and birds. Frequently, 
as in the Cod, the maxillaries and intermaxillaries are so loosely connected, that the 
idea is given of two separate arches ; but, on the other hand, it is to be remembered 
that not only is this looseness of connexion (considered apart from the question of deve- 
lopment, to be afterwards noticed) no reason why they may not be members of one 
segment, but also that the intermaxillaries are always in contact with one another in 
the middle line, while the maxillaries are not so ; that the arrangement is often such as 
has just been mentioned as existing in the Salmon ; and that sometimes the maxillary 
and intermaxillary are even more closely soldered into one arch, of which the inter- 
maxillary forms the proximal, the maxillary the distal part. 
Morjpholofjical Conclusions respecting the Construction of the Facial Segment 
throughout the Vertehrata. 
Ere endeavouring to discover from the above data in what precise manner the maxil- 
laries, intermaxillaries, and nasals unite to form the facial sclerotome, it will be well to 
state distinctly that they cannot be considered as forming in the mammal a continuous 
neural ring behind the nostrils, however tempting at first sight that idea may appear ; 
for such an hypothesis would render inexplicable the union of the intermaxillaries above 
the anterior extremity of the septal cartilage, which occurs in the Dugong, Tapir, &c., 
besides that it could not be applied to other than mammalian forms. 
