506 EXTINCT RODENTS 



to note that some of the extinct genera were much larger than 

 recent forms. At present, Hydroch oerus is the biggest Eodent ; 

 but the genus 3{egamys from the Pampas formation of Argentina 

 was " nearly as large as an ox." The wider range of genera in 

 tlie past is illustrated by Hystrix, which, now an Old-World 

 form, is represented by remains in the Miocene and Pliocene of 

 America. 



It is a significant fact that of living genera Sciurus is the 

 oldest ; for it has been pointed out that in a number of features 

 the Squirrels are among the most primitive of Eodents. The 

 zygomatic arch is slender, and has thus not acquired the specialisa- 

 tion that is to 1)6 found in that part of the skull in other E(xlents ; 

 moreover, the " jugal bone is not suppm'ted Ijy any process from 

 the maxilla exactly as in the primitive Ungulata." The feet, 

 too, are unspecialised, though that is the case with many other 

 genera. It may also be pointed out that the teeth l;)ear not a 

 little likeness to those of Ornitliorltynchus in their multituber- 

 culate character. - 



Some few fossil forms have already been dealt with in the 

 preceding pages. 



The two genera Castoroidcs and Atnbly rhiza, ivoiw the Pleisto- 

 cene of North America and the West Indies, are usually regarded 

 as forming a family. The skull of the former genus indicates an 

 animal of the size of a Bear. It is compared to that of Castor, 

 but it has a wnde infra-orbital foramen. The teeth are four in 

 each jaw, antl are formed of three to five lamellae ; the incisors of 

 this animal are pQwerful but short. Amhlyrhiza, on the other 

 hand, has long incisors which are longitudinally grooved anteriorly. 

 It has a free fibula. This latter as well as other characters have 

 led Tullberff to remove it from association with Gastoroides. 



Order X. TILLODONTIA. 



This group of Eocene mammals is to be defined Ijy a number 

 of characters, of which the more important are the following : — 

 The incisors are enlarged, grow from persistent pulps, and are 

 coated with enamel upon the outer surface only ; they are those 

 of the second pair only, the first and third having disappeared 

 or Ijecome small. The canines are reduced in the later forms. 



