THE SKULLS OF REPTILIA AND AVES. 243 



a ring round the carotid canal.* The alisphenoids ossify in their 

 whole length and breadth ; the orbito-sphenoid only slightly, and 

 the presphenoid not at all. The premaxillary bone arises as an 

 azyffos triangular cartilage between the cornua of the anterior 

 ethmo-vomerine plate. It ossifies from a single centre. 



The auditory capsule, or the future petrosal [— periotic] 

 bone, may, even at the end of this period, be readily separated 

 from the other part of the cranial wall, and still consists, for the 

 most part, of cartilage. On the other hand, the triangular form, 

 which it had before, is not inconsiderably altered, since it greatly 

 elongates forwards, and thus, as it were, thrusts its anterior 

 angle further and further forwards, and becomes more unequal- 

 sided. At the lower edge, or the longer side of it, about 

 opposite to the upper angle, at the beginning of this (third) 

 period, or indeed somewhat earlier, a diverticulum of the 

 auditory capsule begins to be formed (the rudimentary cochlea), 

 and develops into a moderately long, blunt, and hollow appen- 

 dage, the end of which is directed downwards, inwards and back- 

 wards, and also consists of cartilage. Close above, and somewhat 

 behind this ajDpendage, however, there appears, at about the 

 same time, a small rounded depression, in which the upper end 

 of the auditory ossicle eventually rests ; and, somewhat later, an 

 opening appears in this depression w T hich corresponds with the 

 fenestra rotunda of man. Very much later, namely, towards the 

 end of this period, the auditory capsule begins to ossify. Ossifi- 

 cation commences in a thin and moderately long, hook-like 

 process, which is sent forwards and inwards from the lower 

 hollow diverticulum of the cartilage,, and unites with the basi- 

 sphenoid. From this point it passes upwards and backwards, 

 and, for the present, extends so far that, at the end of this 

 period, besides that process, the diverticulum in question, and 

 about the anterior third of the auditory capsule itself, are- 

 ossified, t Later than at the point indicated, an ossific centre 

 appears at the posterior edge of the auditory capsule, where it 

 abuts against the supra- and ex-occipitals, but extends from 

 hence by no means so far forward as to meet that from the 



* These are the " basi-temporals " of Mr. Parker. 

 f This is the pro-otic ossification. 



k2 



