58 MR. E. T. BENNETT ON THE CHINCHILLID, 
distinct lamin, except in the anterior tooth on each side of the lower jaw, is the same 
as in Lagotis ; and the same apparent reversal of the teeth in the two jaws is manifest. 
The posterior lamina of the teeth in the upper jaw, and the anterior in those of the 
lower, is also the smallest of the three, and these smaller damine fall short of the 
respective margins of the teeth, nearly as in Lagotis, giving rise to a similar anomaly 
in the grooving of the inner and outer surfaces of the crowns. But instead of the 
regular curve of the two larger lamine which takes place in Lagotis, those of Chinchilla 
are nearly straight, with the exception of a sharp turn backwards of the inner extremity 
of the intermediate /amina in the upper jaw, and a slight prolongation forwards of the 
outer extremity of the same /amina in the lower. Add to this that a still more marked 
difference occurs in the anterior tooth in the lower jaw, which, instead of being com- 
posed of three distinct lamine as in Lagotis, has the line of enamel subdividing its two 
anterior portions so abbreviated as to extend little more than half way across the tooth; 
which consequently consists of only two distinct lamelle, the anterior bilobate inter- 
nally, but with its osseous substance externally continuous. In all the family the 
lamine of the teeth are directed obliquely backwards and inwards, and still more 
obliquely so in the lower than in the upper jaw. 
From this account of the dentition of the three animals, the differences between 
Lagostomus and the other two will be at once obvious: those which distinguish Lagotis 
and Chinchilla will be better observed in the figures than they can be conveyed in 
words. The following, however, is a summary of the more remarkable points of dis- 
crepancy between them. These consist, firstly, in the curvature of the anterior lamine 
of the teeth of the upper jaw in Lagotis, as compared with the straightness of the same 
lamelle in Chinchilla ; and in the curvature of the middle lamella taking place more 
gradually, and not in the sudden manner in which it occurs in Chinchilla: secondly, in 
the greater extent of the anterior lamella of the three posterior teeth of the lower jaw in 
Lagotis, as compared with Chinchilla: thirdly, in the complete disjunction of the two 
anterior lamine of the anterior tooth of the lower jaw in Lagotis, while in Chinchilla the 
enamel advancing between them from within, extends on the surface of the crown but 
little more than half across the tooth, and thus leaves a space in which the osseous 
portions of the two lamell¢ run into and are continuous with each other. 
I shall conclude with the technical characters of the three genera which constitute 
the family of Chinchillide, and with a few observations on its place in the tribe to which 
it belongs. 
Ordo RODENTIA. 
Tribus Hersrvora, F. Cw. 
Dentes molares eradicati, per totam vitam pulpa persistente crescentes. 
Fam. CHINCHILLIDA. 
Dentes incisores 3, superiores simplices ; molares =, e lamellis osseis binis ternisve 
