170 DINOCERATA. 



At the very base of the Tertiary, we find the cLiss of Mammals well 

 represented by many widely separated groups, which point back to a 

 common ancestry, only in a very remote pei'iod. 



Our j^resent knowledge of the Mammalia, living and extinct, clearly 

 indicates that they must go back at least to the Permian. The generalized 

 mammal of that period, or of still earlier time, was probably quite small, 

 and, in many respects, like an Insectivor. This primitive type would 

 naturally possess all the general characters found in later forms in the 

 various orders of mammals. The characters therefore we should expect to 

 find in this ancestral mammal would be essentially the following : 



(1.) Brain, small and smooth. 



(2.) Teeth, more than forty-four. 



(3.) Vertebrae, biconcave. 



(4.) Trunk vertebra3, more than tliirty. 



(5.) Sacral vertebrae, separate. 



■ (6.) Intercentral bones. 



(7.) Chevron bones. 



(8.) Cervical ribs, free, 



(9.) Clavicles, free.' 



(10.) Coracoids, free. 



(11.) Sternal bones, flat. 



(12.) Humerus with supra-condylar foramen. 



(13.) Feet, plantigrade. 



(14.) Five digits in manus and in pes. 



(15.) Carpal and tarsal bones not interlocking. 



(16.) Separate central bone in carpus. 



(17.) Pelvic bones, separate. 



(18.) Epipubic bones. 



(19.) Acetabular l)ones. 



(20.) Femur witli tliird trochanter. 



(21.) Tln-ee bones in first tarsal row. 



(22.) Astragalus, fiiit. 



(23.) Fibula articulating with calcancum. 



