On Sonic Points in the Structure of the Dicynodont Skull. 345 



Fig. 1 he has the postfrontal carefully marked PtF, and the post- 

 orbital is left unlettered, but in Fig. 4 he has both the postfrontal 

 and the postorbital marked PtF. Probably the lettering of the latter 

 bone is a lapsus calami. The postorbital arch is formed by three 

 bones the postfrontal forming a small part of the upper end, and 

 the jugal forms the whole of the inner side of the lower half. At the 

 upper end of the postorbital arch the postorbital passes backwards 

 on the outside of the parietal and at the posterior end overlapping 

 the squamosal. 



The jugal is a large bone which is developed in three different 

 directions. The anterior part forms the lower wall of the orbit and 

 the greater part of the suborbital arch. It is overlapped by the 

 maxilla and articulates with the lachrymal and with the palatine. 

 It sends up a strong flat process behind the orbit and articulating 

 with the under side of the postorbital portion of the postorbital 

 bone. The posterior process of the jugal extends back nearly as 

 far as the temporal fossa, and lies along the inner side of the 

 zygomatic process of the squamosal. 



The large pineal foramen is situated a little behind the plane of 

 the postorbital arches. It is in the middle of a median bone which 

 has been called the preparietal. This bone is present in most 

 Anomodonts, but is absent in Cisteccphalus. It is unknown in any 

 other order of reptiles. By Newton it is believed to be the inter- 

 parietal, but this is pretty certainly wrong. One might consider 

 whether this is not the true parietal, and the supposed parietals 

 modified post-temporals, but from the condition in Therocephalians 

 and in Cisteccphahis we may safely conclude that the parietals are 

 rightly identified. The preparietal is a narrow bone only a little 

 wider than the foramen, and there is about two-thirds of the bone 

 in front of the foramen and only one-third behind. 



The parietals are a pair of greatly elongated narrow bones. In 

 front they articulate with the frontals, and, passing backwards 

 between the preparietal and the postorbitals, form the median 

 portion of the post-temporal ridge, and articulate with the squa- 

 mosals. Though when viewed from above the parietals are very 

 narrow, this is owing in part to their being covered by the post- 

 orbitals. 



Behind the parietals is a large median bone which is greatly 

 developed laterally, and forms the back of the temporal ridges. 

 This is the bone which has usually been called the interparietal, and 

 whether or not it may be the homologue of one or other of the mem- 

 brane bones of the lower forms, it seems highly probable that it is 



