MR. W. K. PARKER ON THE OSTEOLOGY OF BALiENICEPS REX. 279 



The ali-sphenoids wall-in a great part of the up-turned base of the brain ; the diverging 

 alae of the connate orbito-sphenoids contributing but little to the cranial cavity, whilst 

 the ethmoidal sclerotome is wholly facial and catacentric. 



In the Chick, on the eleventh day of incubation, the cartilaginous ali-sphenoids have 

 coalesced with the cartilage of the base of the cranium (although, from Rathke's obser- 

 vations on the Snake, they are perhaps separately formed at first in the membranous 

 wall), but no osseous deposit has taken place. In the Pigeon, one day after hatching, 

 the centre of this cartilaginous patch is still membranous, and this is the last part to 

 ossify, but it is filled up at the end of the first week. In the Chick, on the fourteenth 

 day , there is a wide ring of bone round the ali-sphenoid, the centre being still cartilaginous. 

 This bone is bounded above by the sphenoido-frontal, in front by the orbito-sphenoid, 

 supero-laterally by the squamosals, below by the basi-sphenoid, and behind by the 

 petrosal. 



Petrosal (PI. LXV. figs. 1 & 7, pt.) 



This latter bone, the petrosal, has a very cellular character externally ; its anterior 

 margin is the foramen ovale, and its posterior termination is indicated by the fenestra 

 ovalis, in which the base of the columella is fixed. Seen through the foramen magnum 

 the petrous bones project considerably into the cranial cavity, passing as thick beams of 

 bone upwards and backwards to lose themselves in the diploe of the combined parietals 

 and epiotics, and downwards and forwards to coalesce with the basi-sphenoids just out- 

 side the posterior clinoid processes. 



Speaking of the petrosal in the young Ostrich, Professor Huxley says (Croon. Lect. 

 p 12), "The par vagum passes out between the bony mass under description and the 

 ex-occipital : the third division of the trigeminal leaves the skull between it and the 

 ali-sphenoid. The portio dura and portio mollis enter it by foramina very similarly 

 disposed to those in the Sheep : superiorly there is a fossa on the inner face of the bone, 

 which corresponds with a more shallow depression in the Sheep, and like it supports a 

 lobe of the cerebellum. Finally, the anterior inferior edge of the bone traverses the 

 middle of the fossa which receives the mesencephalon. In every relation of importance, 

 therefore, this bony mass corresponds with the petro-mastoid of the Sheep, while it 

 differs from il only in its union with the ex-occipitals and the supra-occipital posteriorly, 

 and its contact with the cranio-facial axis below." 



In the Chick, on the eleventh day of incubation, and in the Pigeon, one day after that 

 period, the petrosals are still entirely cartilaginous, the semicircular canals shining 

 through the cartilage, which will be differentiated into these elements, the epiotics, and 

 the mastoid margin of the ex-occipitals. In the Chick, on the fourteenth day, much ossific 

 matter has been deposited, and two days later the relation of these parts can be well seen ; 

 but the most projectingpartsof the semicircular canals still shine through the connecting 

 cartilage. The mass of the petrosal lies internal to, or mesiad of, the ali-sphenoids in 



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