Evolution of Animals 487 



With the above characters now before us, and bearing in 

 mind other structural points of the Amphibia and MammaHa, 

 it would not be difficult to construct an imaginary form that 

 might be temporarily called the amphibio-mammal. If such 

 existed we should expect to find some confirmatory evidence 

 from fossil remains. Unfortunately urodelan and primitive 

 mammalian remains are still rare from their early occurrence 

 in the triassic up to tertiary times when they become more 

 abundant. 



In suggesting a probable arboreal ancestry for the Mam- 

 malia, Matthew (161: 811) considers that a primitive central 

 mammalian type would show the following characters: 



1. Size very small, skull of moderate length, brain case 

 completely enclosed in bone, brain of high type compared with 

 that of Reptilia although lower than in the modern mammals. 



2. Molar teeth "bunodont," i. e., low crowned, the crowns 

 composed of few low broad rounded cones, heavily enamel- 

 covered. 



3. Neck rather short, slender, and flexible, permitting quick 

 and easy turning of the head in all directions. Trunk slender, 

 flexible, ribs rather short and few in number, lumbar region 

 long and comparatively flexible. These features characterize 

 all early tertiary mammals without exception. 



4. Tail very long and flexible with strong muscles toward 

 the base and probably prehensile. 



5. Shoulder girdle of scapula and clavicle. No indications 

 of a separate coracoid have been observed in early tertiary 

 mammals. 



6. Ilia narrow and rod-like, gluteal muscles long and slender. 



7. Upper members of limbs comparatively long and loosely 

 jointed to the trunk, permitting great freedom of motion. 

 The great relative length of the humerus and femur is a striking 

 feature in the limbs of early tertiary mammals. 



8. Ulna and radius separate, equal in size, radius with round 

 head, permitting free supination and pronation. 



9. Wrist and ankle very flexible, all the carpals separate 

 and a centrale present. 



10. Five digits on each foot, the joints permitting of very 

 free motion of fingers and toes, which are tipi)ed by small 

 claws. 



11. First digit more or less apposable in both manus and 

 pes. 



