ON THE VERTEBRATE SKELETON. 225 



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 prefrontals 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 (ante, 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 anterieur' 

 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 73 (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 ante- 

 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 



