THE SKELETON OF THE VERTKHHATE HEAD. 125 



the same bone, including the corresponding halves of the 

 cribriform lamina, and the two halves of the frontal. "We 

 have here, therefore, a centrum, a pair of neurapophyses, and 

 a divided metaneurapophysis. The pair of olfactory nervous 

 centres, which terminate in front the entire series of segments 

 of the neural axis, ai'e the segments of that axis, homologous 

 with this neui-al arch and centmm. In the mammalia only 

 is the upper part of this neural arch expanded and adapted 

 for the protection of the more or less developed fore part of 

 the cerebrum proper. In the central portion and lateral masses 

 of the ethmoid, and in the frontal bones of the mammal, I 

 recognise the centrum and neural arch of a sclerotome, which 

 I provisionally distinguish as the ethmoidal. 



Ccntrimi and Neural Arch of the Ethmoidal Sclerotome in 

 the Osseous Fish. — The more or less concurrent statements of 

 Oken, Bojanus, Geoffroy, Cuvier, and Owen, as well as the 

 relations of the bones themselves, leave no doubt as to the 

 homology of the so-called pre-frontals of the fish. Tliey are 

 ueurapophyseal elements, the lateral ethmoidal masses of the 

 mammal in another form, and minus the ossified olfactory 

 capsules. The median bone superimposed upon the " pre- 

 frontals " of the fish, and which has been very generally held 

 to be the imited nasals, and the spine of the olfactory vertebra, 

 must be homologous with the frontal bone of the mammal if 

 its relations to the " pre-frontals " and olfactor}' nerves of the 

 former are compared with those of the ethmoid and frontal 

 bones, and the olfactory nerves of the latter. Professor Owen, 

 while he adopts the determination of the superior median 

 bone as the united nasals, also holds by the hitherto unanimous 

 opinion of anatomists, that the median bone armed with teeth, 

 situated below the pre-frontals of the fish, is the vomer. 

 Guided by the ethmoid of the mammal, I cannot see in this 

 bone aught else than the homologue of the central element of 

 the mammalian ethmoid. The voincr is a mammalian bone ; 



