420 ANATOMY OF VERTEBRATES. 



angles by the exoccipitalsj, anterior to whicli it attains to the 

 alisphenoids, and is notched externally for the reception of the 

 upper angle of the squamosals. The basisphenoid, a thick hexa- 

 gonal bone, concave from side to side below, nearly flat above, 

 is anchylosed to the alisphenoids : it is perforated or grooved by 

 the entocarotids, but has no clinoid processes nor sella turcica. 

 The alisphenoids, perforated near the middle of their base by 

 the foramina ovalia and rotunda, have a thick quadrate plate on 

 their inner side, forming their cranial surface : they extend into a 

 point anteriorly, and articulate with both the frontal and with 

 the parietal angle of the superoccipital. The neural spine of 

 the parietal vertebra is a thin plate partly detached and partly 

 anchylosed to that of the occipital vertebra : the lower angles 

 are confluent with the diapophyses, called ^ mastoids,' whicli 

 here, as in other Cetacea, are distinct from the petrosals, and 

 chiefly support the squamosals : these enter a groove of the 

 suj^eroccipital posteriorly, and receive the alisphenoid in a 

 groove anteriorly. The presphenoid and the anchylosed orbito- 

 sphenoids form the anterior wall of the cranial cavity, and 

 are perforated by the optic foramina: they articulate with the 

 frontals, sending up a small process into the interspace at the 

 beginning of the frontal suture, which process is impressed by a 

 blind fossa like a small foramen olfactorium on each of its sides : 

 the presphenoid unites with the basisphenoid : the posterior 

 and lateral parts of the orbitosphenoids unite with the alisphe- 

 noids : the fore part of the presphenoid is underlapped by the 

 vomer. There is no cribriform plate. The frontal bones are 

 large triangular plates, concave externally, with the outer and 

 fore angle produced into a superorbital process, the channel on 

 the under part of which contracts as it approaches the cranium 

 into a long, deep and narrow groove, which lodged the muscles of 

 the eyeball. The straight median margins of the frontals are 

 thinned off" and joined by a squamous frontal suture, the right 

 overlapping the left. The whole posterior and lateral border of 

 the frontal, as far as the junction Avith the squamosal, presents a 

 broad, oblique, sutural surface, which joins, by overlapping, the 

 contiguous border of the superoccipital. The smooth cerebral 

 surface of the frontals is flat at the middle, arched at the sides, 

 and not impressed by any convolutions. The vomer expands 

 into two aliform processes at its base, which is applied against the 

 presphenoid and orbitosphenoids ; it then becomes subcompressed 

 and smoothly excavated, but much more deeply at the left side, 

 where it forms the inner and posterior boundary of the single 



