The anterior end of the basitemporal is pierced near the middle line by a pair of holes, the =anterior openings of the Eustachian canals=; while just in front of these and a little further removed from the middle line are the anterior openings of the =carotid canals=. Bristles passed in through the posterior openings of the carotid canals will emerge here (fig. 60, A, 27). In front of the basitemporal the base of the cranium is formed by the =rostrum= (fig. 60, A, 31), or thickened basal portion of the interorbital septum; this bears two prominent surfaces with which the pterygoids articulate. In some kinds of duck these surfaces are borne by well-marked basi-pterygoid processes.
(_d_) _The side of the cranium._ At the base of the posterior end is seen the deep =tympanic cavity=. The dorsal part of this is divided by a vertical partition into two halves; of these the more anterior is the larger, and forms a deep funnel-shaped cavity, the =posterior opening of the Eustachian canal= (fig. 60, B, 26). A bristle passed into this opening emerges through the anterior opening of the Eustachian canal. The more posterior of the two is the =fenestral recess= (fig. 60, B, 28), and is in its turn divided by a slender horizontal bar into a dorsal hole, the =fenestra ovalis=, and a ventral hole, the =fenestra rotunda=. During life the fenestra ovalis lodges the proximal end of the =columellar= chain. Lying at the outer side and slightly dorsal to the tympanic cavity is a deep depression, the =lateral tympanic recess=, and immediately in front of this is the articular surface for the quadrate. The tympanic cavity is bounded below by the basitemporal, posteriorly by the exoccipital, and above by the _squamosal_, a membrane bone, which roofs over a good deal of the side of the cranium, and bears ventrally a prominent surface with which the quadrate articulates. Just in front of this is a large round hole, the =trigeminal foramen= (fig. 60, B, V), behind which the squamosal is drawn out into a short process.
In front of the squamosal there is a prominent forwardly-projecting =postfrontal process= (fig. 60, 8), which ossifies from a different centre from that forming the squamosal, but in the adult is completely fused with it.