417 



croach on the province of the other. The optic nerve passes out 

 more or less in front of the alisphenoid, and behind, or through, the 

 orbitosphenoid. 



The organ of hearing is always bounded in front by the petrosal 

 bone, which limits the anterior moiety of the fenestra ovalis. 



The organs of smell always lie on each side of the ethmovomerine 

 part of the axis. 



The greater part, or the whole, of the petrosal lies behind the 

 centre of the mesencephalon. 



6. The attachment of the mandibular arch to the skull is never 

 situated further forward than the posterior boundary of the exit of 

 the trigeminal ; consequently it cannot belong to any segment of the 

 skull in front of the petrosal. 



But if propositions of this generality can be enunciated with regard 

 to all bony vertebrate skulls, it is needless to seek for further evidence 

 of their unity of plan. These propositions are the expression of that 

 plan, and might, if one so pleased, be thrown into a diagrammatic 

 form. There is no harm in calling such a convenient diagram the 

 ' Archetype ' of the skull, but I prefer to avoid a word whose con- 

 notation is so fundamentally opposed to the spirit of modern science. 



Admitting, however, that a general unity of plan pervades the 

 organization of the ossified skull, the important fact remains, that 

 many vertebrated animals all those fishes, in fact, which are known 

 as Elasmobranchii, Marsipobranchii, Pharyngobranchii, and Dipnoi 

 have no bony skull at all, at least in the sense in which the words 

 have hitherto been used. In these Vertebrata the skull is either 

 membranous or cartilaginous ; or if ossified, the ossific matter pre- 

 sents no regular grouping around a few distinct centres. 



Thus the cranium of the Amphioxus is nothing but a membranous 

 capsule, whose walls are continuous with those of the canal for the 

 spinal cord, and in whose floor lies a continuation of the notochord 

 which underlies the spinal canal. 



In the Marsipobranchii there is a marked increase in the capacity 

 of the cranium as compared with that of the spinal canal, in cor- 

 respondence with the decided differentiation of the cerebral masses ; 

 and, at the same time, the cranial walls have undergone a more or less 

 extensive chondrification. The notochord terminates in the midst of 

 the firm and solid cartilaginous plate which forms the posterior part 



