140 ON THE MOnniOLOGICAL CONSTITUTION OF 



reptile, I cannot conceive how the " pre-frontals," either of 

 the fish or reptile, can be homologous with a mesial bone. 

 Embryologically, I cannot understand how the olfactory nerves, 

 which in the fish and reptiles are situated mesiadof the "pre- 

 frontals," can become placed in the mammal on their outer 

 aspects. The pair of " pre-frontals" in the crocodile or turtle 

 can be legitimately enough conceived as coalescing mesially 

 into a single bone ; but this change presupposes the with- 

 drawal or obliteration of the olfactory nerves ; for, otherwise, 

 two conditions must be admitted, both of which are embryo- 

 logically untenable — first, that the olfactory lobes of the 

 mammal are at one period in its development mesiad to the 

 right and left halves of its central ethmoidal plate ; and 

 secondly, that the nervous and sclerous structures change 

 places, the former passing outwards through the latter, or the 

 latter meeting in front of the former, and passing backwards 

 between them. But the actual facts are these : — The mesial 

 plate, or bar, of the mammalian ethmoid is mesial from the 

 first ; and the olfactory bulbs, or nerves, are situated from the 

 first on its lateral aspects. The mesial plate is the prolonga- 

 tion forward of the central bar of the primordial cranium ; it 

 is a true vertebral centrum, and is continued onwards and 

 downwards into the vomerine portion of the cranial axis. The 

 cribriform lamellae are the only parts, therefore, of the mam- 

 malian ethmoid which present in their embryo and adult con- 

 ditions all the characters of neurapophyseal elements ; con- 

 nected below with their centrum, and laterally or above with 

 their frontal meta-neurapophyses, they, along with the latter 

 and the centrum, close in the fore part of the encephalic portion 

 of the cranial cavity, and enclose the olfactory lobes of the 

 brain. That the olfactory, like the fifth nerve of the mammal, 

 leaves the encephalic cavity by more than one orifice, and that 

 the oKactory " sense-caj)sules" are united to the corresponding 

 neurapophyses, arc circumstances which afford no arguments 



