4 )G Royal Society : — 



fication takes place continuously on each side of the notochord, and 

 beyond it, the two trabecnlse cranii, unhke anything in the spinal 

 column, extend along the base of the cranium. No distinct carti- 

 laginous centra, and consequently no intercentra, are ever developed. 

 The occipital arch is developed in a manner remotely similar to that 

 in which the neurapophysial processes are formed ; but the walls of 

 the auditory capsules, which lie in front of them, and which give 

 rise to some of the parts, most confidently regarded as neurapo- 

 physes by the advocates of the current vertebral theories of the skull, 

 are utterly unlike neurapophyses in their origin. 



So, if we seek for haemal semi-arcs, we find something very like 

 them, arising from the substance of the basis cranii beneath the 

 auditory cartilage ; but there is none connected with the occipital car- 

 tilage, and none with the rudiment of the alisphenoid. The palato- 

 pterygoid cartilage might be regarded as the haemal semi-arc 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 imder 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 Vertebratay 

 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 Vertebratay 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 no- 

 thing 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 exocci[)ital and 

 supraoccipital does, without doubt, })resent a very close analogy to 

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

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

 the train of the exocci})itals ; 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 argumen*, 

 as was supposed by Oken, that the ba^isphenoid, alisphenoid, and 

 parietals, the j)resphenoid, orbrtosphenoids, and frontals reprcjtut 



