292 ON THE STRUCTURE OF THE VERTEBRATE SKULL. 



mass which surrounds the cephalic part of the notochord like a 

 sheath ; the basi-sphenoid, in that part of it which lies between 

 the paired processes (the trabecule) and the anterior end of the 

 notochord ; and the ethmoid (more particularly its body, or pars 

 perpendicularis), in the anterior coalesced part of those two pro- 

 cesses. The body of the presphenoid, on the other hand, is 

 formed below the processes in question, rarely between them. 



" (6.) The parts of the skull just mentioned, however, do not 

 ossify in all Vertebrata with an osseous skeleton, but one, or 

 several, of them sometimes remain cartilaginous, and then grow 

 relatively far less than the others, so that they seem to be pushed 

 aside and suppressed by the neighbouring bones. This holds 

 good especially of the basi-occipital of the Batrachia, and of the 

 basi-sphenoid of these animals and of osseous Fishes. 



lt (7.) The basi-occipital (or, at least, the substance out of 

 which it will become developed) constitutes, originally, like the 

 body of a vertebra, a sheath round a part of the notochord, and 

 the ex-occipitals appear, whilst they chondrify, as outgrowths 

 from the basi-occipital part ; just as the arches of a vertebra, 

 when this is normally developed, appear as outgrowths from its 

 already chondrified body. For the rest, however, the normal 

 development of the occipital bone is quite similar to that of a 

 vertebra, and it therefore may with perfect justice be held to be 

 a cephalic vertebra.* The squama occipitis, which occurs in 

 many, but not in all Vertebrata, and which is not always placed 

 between, but sometimes lies in front of the ex-occipitals, presents 

 no difficulty in the way of this interpretation ; it is an accessory 

 structure, a so-called intercalary bone, the presence of which 

 depends upon the excessive development of the brain. 



" (8.) The two rings, on the other hand, which are formed 

 by the two sphenoids, with the parietals and frontals as their in- 

 tercalary bones, are no longer constructed upon quite the same 

 type as the vertebra. That the alisphenoids and orbito-sphenoids. 

 when they are already chondrified, do not appear to take the 

 form of outgrowths of their centres, but are united with them by 

 membrane, need not, perhaps, be taken very much into account, 



* The Foramina eondyloidea, which occur in the ex-occipitals of many Verte- 

 brata, remind one of the holes of the vertebral arches of the Sharks. 



