246 Genus Ictidopsis. 
Cynodonts, from the Gorgonopsia, and from Cynosuchus. An inter- 
pterygoid vacuity is a feature of the Therocephalia which differs from 
Ictidopsis, however, in many respects—as do the Bauridae. I have 
shown elsewhere that in the development of Whaztsia the loss of the 
interpterygoid vacuity probably took place at a late stage, as a little 
skull possibly ancestral to Whaztsia and from a slightly lower horizon 
differs from the latter type chiefly in the possession of a single molar 
and an interpterygoid vacuity. Jctidopsis is from the Middle Beaufort 
Beds found associated with the zone fossil Lystrosaurus. Nythosawrus 
possibly also first appears near the top of the Middle Beaufort Beds 
but extends through to the Upper Beaufort Beds of Aliwal North. 
By the gradual closing together of the pterygoid plates and the 
consequent elimination of the interpterygoid vacuity Jctidopsis 
TExtT-FIG. 2. 
IcTipopsis FORMOSA, v. Hoep. 
Skull and lower jaw, side view. (Slightly larger than natural size ). 
could conceivably become a typical Nythosaurid; but if this be the 
line of evolution, then we must look for the ancestors of at least some 
of the Cynodonts among forms with an interpterygoid vacuity which is 
divided above by an anterior prolongation of the basisphenoid. Such 
features are seen in the Scalopusauride of the Cistecephalus zone and 
some of the Therocephalia of the Hndothiodon and Tapinocephalus 
zones. In these, however, a suborbital vacuity is persistent ; but the 
ancestors of /ctidopsis and of Nythosaurus may have arisen from a 
Therocephalian in whose descendants the suborbital vacuity gradually 
became obliterated and a secondary palate formed. It has been 
suggested that the Gorgonopsia are further from the line of descent of 
Cynodonts such as Diademodon than has usually been suspected ; and 
the features of Jctadopsis seem to bear out this contention. 
