874 DR. n. UROOM on 



Beds ami the Bnrghersdorp Beds are greatly thinned out, or 

 possibly there is an unconfoi-mity. 



Some years ago I divided the Upper Beaufort, or Triassic Beds, 

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

 Zone, and (3) the Ojpiogiutthios-Zone. While these zones in the 

 main hold good, the limits of them ai-e still unknown. In the 

 extensive Li/sirosaurus beds of Colesburg, Middelburg, and 

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

 M. S. W.atson has recently found Li/strosaurits associated with 

 Cynodonts to the west of Burghersdorp, and a similar association 

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

 sauriis or Cynodont remains are certainly known from the 

 Procolo'phon beds, the Frocolophon-\ike. genus Thelognathus occurs 

 at Aliwal North with Cynodonts, and possibly Procolophon itself. 

 It seems not improbable tliat later work will further subdivide 

 the Ti'iassic Beds, each zone counting fi-om the point where a 

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

 Avith confidence we require to have a much fuller knowledge of 

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



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

 saiirus, Ictidopsis, and probably Galesaurus come from an older 

 zone than the Cynognathits beds. None of the known specimens 

 have been found near Bia'ghersdorp, nor have any traces of 

 Jyystrosaurios ever been found, from which we may conclude that 

 Lystrosaurus and probably these small Cynodonts became extinct 

 before Cynognathus 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 incisors. Five molars are well preserved on 

 the left side and three on the right. 



While in a number of respects the specimen diflfers from 

 Nythosaurxs larvatus Owen, it seems probable from the position 

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

 structure the molars agi-ee with those of Nythosaurus larratits, we 

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



The dentary differs from that of Nythosaurus larvaius in having 

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

 and in the ascending ramus passing more iipwards. 



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

 have three c\isps. 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. 



