506 EXTINCT RODENTS CHAP. 
to note that some of the extinct genera were much larger than 
recent forms. At present, Hydrochoerus is the biggest Rodent ; 
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 Hystriz, 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 Rodents ; 
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 Ornithorhynchus in their multituber- 
culate character. - 
Some few fossil forms have already been dealt with in the 
preceding pages. 
The two genera Castoroides and Amblyrhiza, 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.  Amblyrhiza, 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 Tullberg to remove it from association with Castoroides. 
Order X. TILLODONTIA. 
This group of Eocene mamunals 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. 
