Comparative Morphology of Chordates 



Fig. 551. Pika or cony, Ochotona saxatillis. (Courtesy, American Museum of Natural 

 History, New York.) 



DUPLICIDENTATA under RODENTIA. A Suborder SIMPLI- 

 CIDENTATA then includes all other rodents. 



Distribution: Eastern and Western Hemispheres, mainly north- 

 ern; not native to Australia and Madagascar. 



Under the old name "Edentata," meaning "toothless," were 

 formerly included a small number of mammals whose claim to inclu- 

 sion in the so-called "Order" rested more on some superficial resem- 

 blances than on positive evidence of close genetic relationship — and 

 these "edentates" were not all toothless. The following three Orders 

 contain these animals, most of them in Xenarthra. 



Order 9: XENARTHRA. The Order takes its name from a peculi- 

 arit\ of the articulation between successive vertebrae in the posterior 

 thoracic and lumbar region. The usual articulating processes, a pair of 

 postzygapophyses on one vertebra articulating with a pair of pre- 

 y.ygapophyses on the vertebra next behind it, are present but are 

 supplemented by pairs of accessory articulating processes — a condition 

 not known to occur in any other mammals, but found in some reptiles. 



Other characteristics of the Order are as follows: Sacral region 

 elongated and strengthened by articulation of ischia with several of 

 the more anterior caudal vertebrae. Dentition never complete: no 

 incisors, and no canines unless in some cases they are represented by 

 the most anterior cheek-teeth; small "peglike" cheek-teeth varying 

 from 4 to 10, but in some species 20 or 25, in each half-jaw; adult teeth 

 lack enamel; rapid wear compensated by persistent growth of dentine 



