100 MAMMALIAN DESCENT. [LECT. IV. 



nine, and as few as six. Here we encounter ancient 

 characters, such as undoubtedly existed in prototherian 

 forms, for all normal mammals, from a Man to a Giraffe, 

 have seven joints in the neck. 



As to the limbs, the greatest loss of digits has taken 

 place in the Unau, or Two-toed Sloth. In most of the 

 Edentata the modification has been rather in the form 

 than in the number of the digits. Of course, the skull 

 is the most important part of the framework of these 

 types, as of all others ; this is one of the best corners 

 of the field for the worker, for here is the deepest and 

 richest soil. 



As compared with Marsupials, these types show a 

 skull of a higher order, on the whole ; yet, whilst none 

 of the known Metatheria have lost their teeth, their 

 skull seems to be more archaic than that of the Ant- 

 eaters and Pangolins. In the Armadillos, Sloths, and 

 Aard-vark, the dentition is degraded to such an extent 

 as to put these forms far out of the line of the Mammalia, 

 generally, whilst the teeth are entirely suppressed in 

 the Ant-eaters and Pangolins. This entire absence of 

 teeth, common to two of the groups, and to the Proto- 

 therian Echidna, gives a peculiarly feeble appearance to 

 the face. Also, as in birds, this part is very much 

 drawn out (prognathous) ; in Turtles and Tortoises, 

 where the face is short and steep, the loss of teeth does 

 not seem to modify the head so much as in Birds 

 and Ant-eaters. 



