714 MAMMALIA. 



and may share in the articulation for the lower jaw, that the quadrate is 

 often small, that there is a single temporal arcade comparable to the 

 mammalian zygomatic arch, that the teeth are heterodont, that the 

 pelvic bones unite in an os innominatum, with a continuous ischiac 

 symphysis, that the scapula often has a spine, that the occipital condyle 

 may be double, that there is a beginning of reduction and consolidation 

 of skull bones, and so on. 



But it may quite well be that the Anomodontia are not in the direct 

 line of Mammalian ancestry, but represent a side-branch from transitional 

 forms connecting Reptiles and Mammals. 



The student should look back to the characters common to the 

 Amniota (Reptiles, Birds and Mammals), e.g. the presence of amnion 

 and allantois, the absence of gills, etc., for these indicate a close alliance 

 far apart from Ichthyopsida, and it seems therefore unprofitable to look 

 for the roots of the Mammalian stock so low down as among 

 Amphibians. 



Nevertheless, amid so much uncertainty, we may recall the facts that 

 in Amphibians we find two occipital condyles, a reduced quadrate, a 

 somewhat mammalian carpus, holoblastic ova, and so on. 



The oldest Mammalian fossils are from Triassic strata, but they throw 

 little or no light on pedigree, partly perhaps because they are few and 

 fragmentary, partly also because they seem already specialised forms. 

 They are often grouped together as Allotheria or Multituberculata and 

 placed near the Monotremes. 



In the Jurassic period there are more of the dubious Allotheria, 

 e.g. Plagianlax, some " triconodont " Marsupials, e.g. Triconodon and 

 Ampkilestes, and the Trituberculata, e.g. Atnphitheriiiin, some of which 

 suggest primitive Insectivora. There are few Cretaceous fossil remains 

 of Mammals, but some of the remains suggest that the orders of Eutheria 

 were incipient. 



In the earliest Eocene strata, mammals related to modern types begin 

 to be abundant, but we cannot do more than notice two points 

 (a) there were some generalised types, e,g. Creodonts and Condylarthra, 

 with relationships to several extant orders ; (b) that the early forms were 

 mostly small animals with small brains, pentadactyle, with 44 teeth 

 including small canines and bunodont molars. 



Professor Osborn has suggested that there were two main lines of 

 mammalian evolution (a) the " Mesoplacentalia," c.^'. Amblypoda, 

 Coryphodontia, Dinocerata, Tillodontia, and many Condylarthra and 

 Creodonts, in which the brain remained small and unspecialised, which 

 died out in the Miocene (unless the Marsupials, Insectivores, and 

 Lemurs represent their descendants), and (b) the successful lines of 

 " Cenoplacentalia," which made so to speak a fresh start, with a 

 premium on brains, and led to most of the modern types. In almost 

 every case, it may be said that an order begins with small representatives, 

 and that the giant forms almost always indicate the end of a race. 



