546 ORIGIN OF MAMMALS xix. 7 



and nocturnal; few remains of these are found as fossils, but dis- 

 coveries now made have helped to bridge the gap (Kermack and Mus- 

 sett, 1958). No doubt many types existed of which we have no fossil 

 remains, but the Mesozoic mammals that we know can be divided 

 into five orders, *Docodonta, *Multituberculata, *Triconodonta, 

 # Symmetrodonta, and *Eupantotheria (= *Trituberculata). The last of 

 these may perhaps be directly related to the animals that gave rise to 

 the modern mammals, the other four lines are specialized offshoots. 

 All of them were true mammals in that the articulation of the lower 

 jaw was between dentary and squamosal, though there is evidence that 

 the articular and other bones remained relatively large in the doco- 

 donts. The brain-case (where known) was high and the temporal 

 fossa joined with the orbit. The post-cranial skeleton is little known 

 and indeed many forms are known only from lower jaws. However, 

 the pectoral girdle of docodonts includes the same bones as that of 

 monotremes in an even more reptilian form. This and other features 

 suggest that the monotremes have evolved from animals like the 

 *Docodonta and that they have preserved to the present day many 

 features that characterized the mammalian stock during the Mesozoic 

 period. Some of the 'Mesozoic mammals', however, had advanced 

 beyond the stage of the monotremes, for instance the limb girdles of 

 multituberculates resemble those of modern mammals rather than 

 reptiles. 



For purposes of classification we can most conveniently divide the 

 class Mammalia into four subclasses, the Eotheria for the docodonts, 

 Prototheria for the monotremes, Allotheria for the multituberculates, 

 and the Theria for the modern mammalian line. This latter subclass 

 can then be subdivided into three infraclasses, Pantotheria for those 

 'Mesozoic mammals' that probably led to the rest, Metatheria for the 

 marsupials, and Eutheria for the placentals. The triconodonts must 

 be left as of uncertain affinities. This classification is based upon that 

 provided by G. G. Simpson of the American Museum of Natural 

 History, who has not only greatly increased our knowledge of many 

 orders of mammals but also provided a complete systematic review of 

 the group. 



The *Docodonta are known from isolated teeth and jaws in North 

 America and more abundant remains from the latest Triassic of South 

 Wales, which include parts of the skull and shoulder girdle. The molar 

 teeth carry three cusps in a row, a condition known as 'triconodont'. 

 There is a well-marked condyle on the dentary and on the lower margin 

 of the jaw a process (the 'angle'). Above this is a conspicuous trough, 



