S'i'-h DR. R. BUOOM OX 



Betis anrl tiae Burgiiersdorp Beds ai'e greatly thinned out, or 

 po.s.sibly there is an unconformity. 



Some yeai's ago I divided the Upper Beaufort, or Ti'iassic Beds, 

 into three zones : (1) the Li/stroscmrus-Zone, (2) the Procolophon- 

 Zone, and (3) the Cynoguathus-TiOWQ, While these zones in the 

 main hold good, the limits of them are still unknown. In the 

 extensive Li/strosauruc beds of Colesburg, Middelburg, and 

 Cradock no Procolophons or Cynodonts are known ; but Mr. D. 

 M. S. Watson has recently found Lystrosaurus associated with 

 Cynodonts to the west of Burghersdorp, and a similar association 

 we now know occux-s at Harrismith. Again, while no Lystro- 

 saurus or Cynodont remains ai^e certainly known from the 

 Procolophon beds, the ProcQlophon-V\k.Q genus Thelognathus occurs 

 at Aliwal North with Cynodonts, and possibly Procoloplion itself. 

 It seems not improbable that later work will further subdivide 

 the Triassic Beds, each zone counting from the point where a 

 new type begins, but before we can make any further advance 

 with confidence we require to have a much fuller knowledge of 

 the distribution of the fossils of the Upper and Middle Trias. 



In the meantime I think we are safe in stating that Nyilio- 

 sauyms, Ictidopsis, and probably Galesaurus come from an older 

 zone than the Gynognathus beds. None of the known specimens 

 have been found near Burghersdorp, nor have any traces of 

 Lystrosaurus ever been found, from which we may conclude that 

 Jjystrosaurus and probably these small Cynodonts became extinct 

 before Gynognathus appeared. 



Nythosaurus browni, sp. n. (PI. XCIII. fig. 23.) 



This new species is founded on an imperfect lower jaw obtained 

 by Mr. Alfred Brown at Aliwal North. A large part of both 

 dentaries is present, but most of the symphyseal portion is missing 

 with the canines and incisoi-s. Five molars are well preserved on 

 the left side and three on the right. 



While in a number of respects the specimen differs from 

 Nythosaurus larvatus Owen, it seems probable from the position 

 of the symphysis that there were seven molars, and as in general 

 structure the molars agree with those of Nythosaurus larvatus, we 

 may consider it as probable that the species belongs to this genus. 



The dentary differs from that of Nythosaurus la7-vatus in having 

 a more slender horizontal ramus, in having a more marked angle, 

 and in the ascending ramus passing more upwards. 



The molars preserved are probably the 3rd to the 7th. All 

 have three cusps. What is apparently the 3rd molar has the 

 median cusp short, and the others only feebly developed. In the 

 4th and the other later molars the middle cusp is about twice as 

 long as the anterior and posterior cusps. In the second last 

 molar there is an additional small anterior cusp which gives it 

 four cusps, but there appear to be only three cusps in the last 

 molar. The five molars occupy 13'5 mm. 



