ON THE THEORY OF THE VERTEBRATE SKULL 583 



of the presphenoidal region, though the grounds for so doing are not 

 very strong; but the premaxillary cartilage is something quite without 

 parallel in the spinal column. 



4. The mode of ossification of the skull, and the ultimate arrange- 

 ment of its distinct bony elements, are at once curiously like, and 

 singularly unlike those presented by the spinal column. The basi" 

 occipital is ossified precisely after the manner of a vertebral centrum. 

 Bony matter is deposited around the notochord, and gradually extends 

 through the substance of the cartilaginous rudiment of the part. 



The combined basi- and pre-sphenoid in Pisces and Amphibia is 

 an ossific deposit, which takes place on the under surface of the basal 

 cartilage in front of the basioccipital, and extends thence completely 

 beneath the pituitary interspace as far as the ethmoid. It might be 

 paralleled by the subchordal ossification in the coccyx of the frog, or 

 by the cortical ossification of the atlas in many higher Vertebrata, if 

 it really underlay a portion of the notochord ; but at the very utmost 

 the notochord only extends into its posterior extremity. 



In some of the higher Vertebrata, as the snake, the osseous basi- 

 sphenoid arises in the substance of its cartilaginous rudiment, while 

 the osseous presphenoid underlies its cartilage. In others, both bones 

 appear to arise directly in their cartilaginous forerunners. But 

 nothing can be more irregular than the mode of ossification of the 

 presphenoid, ethmoid and vomer in the vertebrate series, or less like 

 the very constant and regular course of ossification of true vertebral 

 centra. 



With respect to the ossification of the lateral and superior con- 

 stituents of the skull, the development of the exoccipital and 

 supraoccipital does, without doubt, present a very close analogy to 

 that of the separate pieces of the neural arch of some vertebrae in, 

 e.g., a crocodile. The alisphenoids and orbitosphenoids follow in 

 the train of the exoccipitals ; but I know not where in the spinal 

 column we are to find a parallel for the double parietals and frontals. 

 But waiving this difficulty, and supposing, for the sake of argument, 

 as was supposed by Oken, that the basisphenoid, alisphenoid and 

 parietals, the presphenoid, orbitosphenoids, and frontals represent 

 the elements of two vertebral centra and neural arches, what is to be 

 made of the petrous and mastoid bones ? 



The difficulty has been eluded by terming the petrosal a " sense- 

 capsule," the mastoid a " parapophysis." But I apprehend that 

 neither of these explanations can be received for a moment by those 

 who are acquainted with the development of the skull, or with the 

 true homologues of the bones in question in the vertebrate series, or 



