378 The Extinct Rodentia of North America. [April, 
cates great power of extension of the femur, but whether this ex- 
tension was effective in running or 
kicking is uncertain. The absence of 
tibial crest, and the shortness of the 
foot militate against the supposition 
that these animals possessed powers 
of leaping, and their swimming pow- 
ers would be impaired by the same 
structural characters. These chat- 
acters of the posterior limb in Am- 
blyrhiza are very peculiar, and arè 
no doubt connected with some 
_ peculiarity of habits which we have 
yet to ascertain. 
This genus clearly enters Pro- 
fessor Brandt’s division of the Ro- 
dentia, which he terms the Hystri- 
paR The evidence is seen 
primarily in the free fibula and in 
the development of the angular por- 
tion of the mandible on the ex 
side of the incisive alveoli; the | 
Fic. 29.—Posterior foot of Ambly- small coronoid process and 
thiza, “two thirds nat. size, Fig. a 
tibio-tarsal articulation of A. Zatidens generic characters add to the weig! 
nats Pc FA a Fisa ae A sat as- of the evidence. Mr. E. E. R. ee! 
external side; é, second row of tar- has recently published a very 
A ats out ics able resumé of the, Steg 
er side. From Anguilla. Original. of the subdivisions, including 
genera of the Rodentia. He of 
vides the Hystricomorpha into numerous families, some 0 
which, at least, appear to the writer to rest on rather slender base 
In the comparison with Amblyrhiza, the Hystricide and Day” 
proctidæ may be dismissed, from the fact that their molars bi 
not divided transversely by laminz of enamel. The compart an 
is with the Chinchillide and Caviide, The molar dentition * 
that of the former family, but the absence of a masseteric fi 
separates it from the genera arranged by Mr. Alston ni 
Caviide, although I cannot perceive that such a character sh pent : 
define a family group. The incisors of both these groups 
called by Mr, Alston “short.” I have shown those of Ambi 
