STRUCTURE 30 1 



any diagnosis is lialjle to be faulty, even assuming that they are 

 a homogeneous group. To avoid confusion, we characterise the 

 Theromorpha as R&ptiles icith a firmly fixed quadrate, a single 

 temporal arch, an interparietal foramen, and a 2Jelvis in which 

 the 2^1(^)68 and ischia form one stout, ventral ST/nqyhysis. 



The dentition is most abnormal, and permits the division of 

 the Theromorpha into two or three main groups. In the Pareia- 

 sauri the teeth of the upper and lower jaws form rather even 

 series of nearly equal size ; smaller teeth are carried by the 

 palatal bones. In the Theriodontia the teeth are differentiated 

 in a truly Mammalian fashion into incisors, prominent canines, 

 and multicuspid or tubercular molars. Each tooth, and this 

 applies to all Theromorpha, is implanted in a separate alveolus : 

 Tritylodon only seems to have double-rooted molars. The lower 

 canines cross in front of the upper, just as in Mammals. In 

 Placodus, which probably belongs to this assembly, the teeth are 

 few in numbers, very broad and flat, especially those of the 

 palate. In Dicynodon and Gordonia the teeth are restricted to 

 a pair of conical, sometimes very large, tusk-like upper canines, 

 and in Oucfenodon the whole mouth is toothless. 



The configuration of the skull shows two main types. In 

 the Pareiasauri it is completely roofed in by dermal bones, the 

 only holes on the sm-face being the nostrils, orbits, and the 

 interparietal foramen. 



The most striking feature of the second type of skull is the 

 tendency to form an almost Mammalian zygomatic arch by the 

 junction of the much elongated squamosal with the jugal bone, both 

 abutting against a downward process of the postfrontal bone. 

 The skull shows a pair of wide supratemporal foramina bordered 

 by the parietals, squamosals, and postfrontals. The composition 

 of the temporal arch varies considerably in detail, and in Cyno- 

 (jnathus crateronotus at least there is a small hole within the 

 arch, between the squamosal and jugal, probably the last 

 remnant of the otherwise absent infratemporal foramen. Except 

 in the roofed-in skulls of Pareiasaurus and Elginia there is no 

 separate quadrato-jugal element. The quadrate is firmly fixed by 

 the overlapping squamosal, and the whole pedicle for the support 

 of the mandible is rather elongated, and either stands vertically 

 or slants forwards. The mandible itself is compound. The 

 pterygoids extend backwards so as to approach or reach the 



