On Some Points in the Structure of the Dicynodont Skull. 343 



The lachrymal has a comparatively small facial portion, though it 

 forms a considerably larger part of the orbital wall. The foramen is 

 rather large and lies inside the orbit. 



The pre frontal is a little larger than the lachrymal, and forms the 

 anterior part of the supraorbital ridge. It also forms a considerable 

 portion of inner Avail of the orbit. 



The frontals are paired elongated bones which form most of the 

 interorbital region, and pass back as far as the pineal foramen. 

 Though the suture between the frontals and the nasals is not very 

 clearly denned, owing to the thickening of the bones, it is probably 

 very nearly as determined by Seeley. Posteriorly the frontals 

 become very narrow as they pass back between the preparietal and 

 the postfrontal and postorbital. The suture between the frontals and 

 the parietals is on the plane of the front of the pineal foramen. The 

 line indicating a supposed suture in front of this in Fig. 1 is an error. 

 Where the frontals meet each other there is a slight median ridge, 

 and the orbital margins are considerably elevated, leaving a pair of 

 fairly deep furrows along the frontal bones. On the upper side of 

 each bone opposite the middle of the orbit are a pair of short grooves 

 which end in foramina, most probably for branches of the supra- 

 orbital branch of the Vth nerve. 



The postfrontal bones can be very clearly denned for the first 

 time in a Dicynodon or Oitdenodon skull. They were known to occur 

 iu Lystrosaurus, in a number of Therocephalians and in Dino- 

 cephalians and Pelycosaurs, but they are lost in all Cynodonts, 

 .and were hitherto believed to have been also lost in Dicynodon and 

 Oudcnodon. The part that now proves to be a distinct postfrontal bone 

 was hitherto regarded as a part of the frontal, the suture being 

 usually indistinct or lost. In Fig. 1 the two postfrontals are well 

 shown as small triangular bones situated at the posterior and upper 

 corner of the orbit. On the posterior side of each is the postorbital 

 and on the inner side the frontal. In my opinion the postfrontals 

 extend further back than indicated in Fig. 1, as delicate processes 

 by the side of the frontals. 



The postorbitals are large bones which form almost the whole of 

 the inner wall of the temporal fossa and the greater part of the post- 

 orbital arch. This large bone used to be regarded as the postfrontal 

 (Seeley 1889, Broom 1901, 1903), but the discovery of a small but 

 distinct bone in front of it in ScylacQsanrus in 1903 showed that 

 the large bone must be called the postorbital, the small more 

 anterior one being manifestly the postfrontal. Exactly what 

 Seeley's position in the matter was latterly I cannot say. In 



