So6 EXTINCT RODENTS 



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

 recent forms. At present, Hydrochoerus is the higgest Eodent ; 

 but the genus Megamys from the Pampas formation of Argentina 

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

 the 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 Rodents. The 

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

 tion that is to be found in that part of the skull in other Eodents ; 

 moreover, the " jugal bone is not supported by 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 bear not a 

 little likeness to those of Ornithorliynchv.s in their multituber- 

 culate character. 



Some few fossil forms have already been dealt with in the 

 preceding pages. 



The two genera C'astoroicles and Amhlyrhiza, from 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 wide infra-orbital foramen. The teeth are four in 

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

 this animal are powerful but short. AmMyrhiza, 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 Tullbercr to remove it from association with Castoroides. 



Order X. TILLODONTIA. 



Tliis group of Eocene mammals is to be defined by 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 become small. The canines are reduced in the later forms. 



