GENERAL CHARACTERS OF THE MAMMALIA. 285 



gressive specialisation can be traced as we approach the pres- 

 ent day. Thus the Eocene Mammalia very commonly pos- 

 sessed the full typical number of twenty-two teeth in each 

 of the jaws, and these teeth were more uniform in size, less 

 conspicuously differentiated into groups, and more closely 

 approximated to one another than in most of the later forms. 

 In the older forms, further, the molars were usually short- 

 crowned, and we can trace a progressive lengthening of the 

 crowns of these teeth, in the course of time, this condition- 

 ing a greater capacity to resist attrition, and a consequent 

 suitability in their possessor for a more prolonged Kfe. In 

 other cases, we can trace a gradual and progressive stunting 

 or abortion of the lateral digits of the typical five-toed limb, 

 accompanied by a correspondingly progressive elongation and 

 strengthening of the central and remaining digits. Lastly, 

 it can be shown that there has been in many instances a pro- 

 gressive increase in the size of the brain, as we approach the 

 present day. Most of the Eocene Mammals, in which the 

 cranium is known, possessed brains of very small size in 

 proportion to the bulk of the body ; and this disproportion 

 gradually lessens as we pass through the Miocene and Pli- 

 ocene to the Eecent period. 



In the following are given the characters of each order of 

 the Mammalia, with the range in time, and, so far as known, 

 the more important fossil forms of each. The number, how- 

 ever, of known fossil Mammals is so great, and in many cases 

 they exhibit so many peculiarities and divergences from ex- 

 isting forms, that nothing more can be attempted here than 

 to give a brief and general sketch of the palfeontological 

 history of the class ; attention being drawn, where it may 

 seem necessary, to extinct types of special interest. 



