III. PHYLOGENY AND RELA- 

 TIONSHIPS 



Six orders of mammals are known to have lived in the Mesozoic : 



1. Multituberculata, Triassic to Eocene. 



2. Triconodonta, Jurassic. 



3. Symmetrodonta, Jurassic. 



4. Pantotheria, Jurassic. 



5. Marsupialia, Cretaceous to Recent. 



6. Insectivora, Cretaceous to Recent. 



Since the first Mesozoic mammal was discovered to the present day, there has 

 never been any real agreement among students as to the relationships of these various 



orders among themselves or to later 



I 

 I 



mammals, or even as to the real existence 

 of the various groups as such. This lack 

 of any approach toward unanimity, even 

 as regards the very fundamentals of the 

 whole problem, has been partly due to 

 the faulty nature of the known materials. 

 The inadequacy of knowledge has been 

 so deeply felt that, in general, students 

 of mammals, aside from two or three 

 specialists, have not even felt it necessary 

 to attempt to grasp the true scope and 

 meaning of what has been known. Atten- 

 tion has chiefly been focused on the last 

 third of mammalian history without 

 troubling the obscurity of the first two 

 thirds. 



This memoir and its companion vol- 

 ( Simpson 19283) should show, 



ume 



Fig. 56. Upper molars in the six Mesozoic orders 

 of mammals. A, Triconodonta, Priacoion. B, Mul- 

 tituberculata, Ctenacodon. C, Symmetrodonta, 

 Eurylambda. D, Pantotheria, Melanodon. E, Mar- 

 supialia, Pediomys. F, Insectivora, Gypsonictops. 

 Not to scale. 



however, how much more extensive our 

 knowledge really is than would be sup- 

 posed after a perusal of any general 

 works on mammals, recent or fossil. The 

 specimens at our disposal could be in- 

 creased a thousand-fold and still fail to 



