THE SKULL OF FISHES. 81 



oblongata, cerebellum, optic lobes, pineal sac, and cerebral hemi- 

 spheres, but also the olfactory lobes. The lateral cartilaginous walls 

 of the cranium are continued forwards from the acoustic capsule 

 between the basal and superior osseous plates : the part perforated 

 by the fifth pair of nerves, and protecting the side of the optic lobes, 

 represents the great ala of the sphenoid : the next portion in advance, 

 protecting the sides of the cerebral hemispheres and perforated by 

 the optic nerve, answers to the orbital ala of the anterior sphenoid : 

 and the cartilage terminates by a part which is perforated by the 

 olfactory nerve, and which abuts laterally against the ascending or 

 palatine process of the maxillary arch. 



The outward extension of the lateral cartilages of the cranium 

 downwards, in the form of a broad triangular plate, the apex of 

 which forms the articulation for the lower jaw, is like that which we 

 see in the Chimaera ; but ossification has extended along two tracts, 

 which converge as they descend, one (yfig. 26. 28) from behind to the 

 outer, the other {ih. 23) from before to the inner side of the carti- 

 laginous maxillary joint, which these bony plates strengthen and 

 support like the backs of a book. The posterior of these bony arches 

 is obviously the homologue of the tympanic pedicle in the Squatina : 

 the anterior bony arch as plainly answers to the pterygoid buttress in 

 osseous fishes ; but it is here confluent with the coalesced palatine 

 and superior maxillary bones, the dentigerous part of which extends 

 outwards, downwards, and backwards (yfig- 29. 21), but does not 

 29. reach, as in the Shai"ks and Rays, the mandibular 

 joint. From the upper part of the palato-maxillary 

 portion a compressed sharp process {ih. 2o) ascends 

 obliquely backwards, and terminates in a point : the 



Cranial spines and . ■ -, r^ ■, ■ -ii itit 



upper jaw of /.fpi- inner Side oi this process is closely attached by liga- 

 ment to the fore and outer part of the frontal por- 

 tion of the epicranial bone {ih. ii) ; the outer side of the process is ex- 

 cavated for the reception of the outer and anterior process of the 

 remarkable bone, which in my Memoir (xxxiii. p. 334.) I have 

 compared with the post-frontal bone. This bone {Jig. 27. 12), in con- 

 nection with the ascending process of the maxillary {ih. 20), forms the 

 upper part of the orbit, and behind this connection it sends out the 

 post-orbital process, beyond which it extends backwards, freely over- 

 hanging the fronto-occipital, and gradually decreasing to a point, which 

 terminates just above the occipital spine, in the position of the mas- 

 toid, in bony fishes, and giving attachment to the anterior end of the 

 great doi-so-lateral muscles of the trunk. This bone is flat above like a 

 scale, and from its superficial position might be classed, like the similar 



VOL. H. G 



