F 
ee 
ON THE VERTEBRATE SKELETON. 925 
perforated by the olfactory nerves (the course of which along the rhinen- 
cephalic continuation of the ‘cranial cavity, is shown by the arrows, ol. 14, 
figs. 8 and 23) prior to their final expansion on the olfactory organ; the 
main body of the bone forms the fore-part of the interorbital septum and 
the back part of the nasal septum, a slight outstanding ridge or angle 
dividing the two surfaces: it rests below upon the rostral prolongation of 
the presphenoid, which, however, barely divides it from the semicylindrical 
grooved vomer (13) which sheathes the under part of that process. The 
posterior extremities of the palatines develope broad horizontal plates mesiad 
and upwards (fig. 23, 20), which join the lower border of no. 14, where it rests 
upon the presphenoid. The outer margins of the anterosuperior expansion 
of no. 14 come into contact with the lacrymals: the posterior border of the 
vertical or rhinencephalic plate joins and soon coalesces with the orbitosphe- 
noids (10). Thus we have all the essential characters of the prefrontals in 
the fish, the frog and the crocodile, with a repetition of their first important 
modification in the tail-less batrachians, viz. that of median confluence ; and 
it is not unimportant to observe that this is associated with the obliteration of 
other cranial sutures, by which also those batrachians resemble birds. The 
first step in the progress of this median approximation of the prefrontals, is 
the development of the plates which, in certain fishes, convert the olfactory 
grooves into foramina; these mesial plates next come into contact at the middle 
line, e. g. in Xiphias and Ephippus ; they proceed to coalesce in the frog, and 
the pretrontals are so much further compressed in the bird that the olfactory 
grooves open upon the outer or lateral instead of the inner or mesial surfaces of 
the rhinencephalic plates: they are, however, very deep grooves in the ostrich. 
and in the apteryx are canals protected by a distinct external-plate. The 
interruption of the direct vomerine connection by the prolonged presphenoid 
is the chief secondary modification of the prefrontals in the bird. No other 
bone in the bird’s skull repeats the more essential characters of the prefrontals 
in fishes and reptiles, save the bone no. 14, figs.8 and 23. Cuvier calls this bone 
the ‘ethmoide’; but blames the clear-sighted and consistent German anato- 
mists who applied that name to the prefrontals in fishes and reptiles ; yet the 
part of Cuvier’s ethmoid in the bird answering to the ‘ lamina cribrosa’ of the 
mammal, sometimes gives passage to the olfactory nerve by a single foramen, 
sometimes by merely a groove, a difference which does not prevent him 
adopting the homology here, though he opposes it to the adoption, by 
Bojanus, of the homology of the same part in the fish (ande, p. 215). The 
smooth plate forming, with the orbitosphenoid, the interorbital septum, is 
the ‘os planum,’ or papyraceous plate of the bird’s ethmoid, with Cuvier : 
the masking of this part in most mammals by the downward development 
of the orbital plates of the frontal, offered no difficulty to the ethmoidal de- 
termination of no. 14 in the bird; and it forms as little valid objection to 
Oken’s mode of expressing the ethmoidal homology of the prefrontals in the 
cold-blooded ovipara. 
For the reasons before assigned, viz. that the terms ‘ frontal antérieur’ 
had been given to the bone in question, no. 14, in those animals in which it 
deviates least from its general type, as the nasal neurapophysis, I retain the 
name prefrontal for it under all its metamorphoses. Cuvier, after balancing 
the characters of the bones nos. 15, 22 and 7a (fig. 23) in birds, inclines to the 
opinion that 15 is the true nasal, and 22' an essential part (nasal process) of 
the premaxillary : with regard to 73, he says, “les os externes et plus voisins 
de l’orbite seraient presque comme on le voudrait, ou des frontaux anté- 
rieurs ou des lacrymaux.” In which case, no. 14 having been described as 
the ‘ethmoid,’ one or other of the above-named bones would be wholly absent 
