CLASSIFICATION, AND PHYLOGENY OF THE DINOENITHID^. 387 



ridge already mentioned (p. 385), and finally ends in a short, h\\mt joretympanic process, 

 situated immediately in advance of the outer end of the quadrate facet. The ventral 

 portion of the posterior limb of the tympanic ridge, between its junction with the 

 posterior tympanic ridge and its termination in the pretympanic process, is conveniently 

 distinguished as the inferior tympanic ridge {inf.tymp.r.). Between the inferior and 

 posterior ridges thus defined, a more or less triangular space is enclosed which serves 

 for the origin of the second portion of the temporal muscle, and may be distinguished 

 as the posterior tympanic fossa {post.tympfos.). Lastly, the main temporal fossa is 

 imperfectly divided, in some species, by a vertical mid-temporal ridge (figs. 12 & 13, 

 m.temp.r.) at about the junction of its anterior and middle thirds and just over the 

 lateral portion of the coronal suture. 



The temporal fossa varies considerably in width in the difierent species, being 

 narrowest proportionally in Mesopteryx casuarina, widest in Anomalopteryx didiformis. 

 But its most striking feature is the difierent degree of its extension on to the cranial 

 roof; in most species the distance between the right and left ridges is but little less 

 than the diameter of the cranium in the temporal region, while in Anomalopteryx 

 didiformis it is hardly more than half that dimension. Moreover, there is usually a 

 flat area between the temporal and lambdoidal ridges (see Plate LXI.) ; but in 

 Anomalopiteryx didiformis (fig. 24), Binornis, species a, and Pachyornis, species a 

 (fig. 26), in all of which the temporal fossa is unusually large, the two ridges are in 

 contact. In Pachyornis elepliantopus, in which there is a wide area between the two 

 lambdoidal ridges, the anterior one often touches the temporal before uniting with the 

 posterior ridge. 



The portion of the dorsal region of the temporal fossa lying behind the mid-temporal 

 ridge is formed, as already stated, by the parietal, the part in front of the ridge by the 

 frontal (figs. 7 & 18) ; its ventral region is furnished by the alisphenoid, which, in the 

 young, joins the parietal by a straight horizontal suture situated at about the level of 

 the roof of the tympanic cavity. Below, the alisphenoid is separated from the basi- 

 sphenoid, in the youngest cranium I have seen, by a gently curved suture (fig. 18) 

 extending forwards and downwards from the trigeminal foramen. 



The trigeminal foramen (foramen ovale, Owen, trig for.), for the second and third 

 divisions of the fifth nerve, is situated in the side-wall of the cranium below the 

 temporal fossa, and in the same transverse plane as the posterior edge of the 

 basipterygoid process (figs. 3 & 13) ; it is bounded above by the alisphenoid, below by 

 the pretemporal wing. Sometimes it is partially or completely divided into two 

 passages by a horizontal bar of bone ; this duplication of the foramen seems to be 

 constant in Mesopteryx casuarina. About 1 cm. below and in front of the trigeminal 

 foramen is a small [qti. arterial ■?) foramen situated just above the base of the basi- 

 pterygoid process as in Apteryx. 



The orhits are smaller than in the majority of birds, and, as Owen pointed out, there 

 can hardly be said to be an interorbital septum, owing to the backward extension of 



