386 PEOF. T. JEFFEKT PAEKER ON THE CEANIAL OSTEOLOaT, 



by the paroccipital process, the latter bemg directed slightly backwards. In Emeus 

 (figs. 38, 42, and 46) the two form a slightly acute angle, the paroccipical process 

 being directed forwards ; in E. crassiis there is often a kind of step at the junction of 

 the two, the margin of the cavity being therefore sinuous. In Ammalopteryx (fig. 36) 

 the paroccipital is still more sharply inclined forwards, the dorsal and posterior edges 

 of the tympanic cavity meeting at a very acute angle, and the width of the cavity from 

 before backwards being greatly reduced. 



On the roof of the tympanic cavity is the large obliquely-transverse articular facet 

 for the head of the quadrate ; its outer end is immediately mesiad of the posterior 

 temporal fossa described below, and is bounded behind by the zygomatic process ; from 

 this point it passes inwards and backwards, its inner end being exactly opposite the 

 Eustachian groove. According to my observations the inclination of the quadrate facet 

 (see Lydekker, ii, p. 298) is a point of no classificatory value; a straight line drawn 

 through the axis of the facet makes an angle with the sagittal plane, which rarely sinks 

 below 130° or rises above 140°, and which is, moreover, very variable, sometimes 

 differing considerably on the two sides of the same skull. The outer two-thirds of the 

 facet is furnished by the squamosal, the inner third in about equal proportions by 

 the exoccipital and pro-otic (Plate LXIl. fig. 70). 



The zygomatic process is, as it were, bent round the quadrate facet so as to bound its 

 outer end posteriorly as well as externally, and its flattened posterior face is continued 

 backwards into a horizontal tympanic ledge (fig. 2), the inner border of which gives 

 attachment to the tympanic membrane, while externally it is produced into a rough 

 horizontal sxipratympanic ridge. 



Immediately posterior to the quadrate facet is a large pneumatic foramen leading 

 into the diploe of the pro-otic and squamosal. Mesiad of the inner end of the facet 

 are usually three more or less well-marked depressions in the roof of the tympanic 

 cavity ; of these, the one nearest to the quadrate facet is a pneumatic foramen ; the 

 hindermost of the other two is a simple depression in the bone, while the foremost is 

 the fenestra! recess, containing the fenestra ovalis and fenestra rotunda. Just anterior 

 to the recess, and separated from it by a vertical bar of bone, is the small aperture for 

 the facial nerve, immediately dorsad of which is a large pneumatic pit. There is 

 another small but deep pneumatic foramen encroaching on the anterior border of the 

 quadrate facet, and the whole wall of the tympanic cavity is honeycombed with less 

 constant depressions of various sizes. 



The temporal fossa is a deep depression between the postorbital process in front 

 and the squamosal prominence behind. It is limited above by a strongly marked 

 temporal ridge (hnea semicircularis, temp.r.) which marks the origin of the temporal 

 muscle ; the ridge forms an even curve, passing from the posterior edge of the post- 

 orbital process, at first backwards and upwards, and then curving round and passing 

 downwards and forwards (figs. 3 and 13), so that the whole temporal fossa has a strong 

 backward slope ; at the base of the zygomatic process it joins the posterior temporal 



