MR. E. T. BENNETT ON THE CHINCHILLID. 57 
ticulars with Lagotis, and differing from Chinchilla. The sacral vertebre are stated to 
be three in number ; but in the plate, two only appear to be united to the ossa ilii by 
the sacro-iliac symphysis, as in the other known animals of the family. Making this 
allowance, the number of caudal vertebre is twenty-one. The anterior extremities have 
little to distinguish them from those of Lagotis, with which they exactly correspond in 
the number of the toes: as in Lagotis, there is no vestige of a thumb, The posterior 
limbs bear the same proportion to the anterior, being just double their length ; but the 
number of the metatarsal bones, and consequently of the toes, is only three, and the 
claws, especially the middle one, are much larger, stronger, and more produced. In 
this particular the figures given by Mr. Brookes are defective, as exhibiting the claws 
far smaller and more curved than is natural. 
I now come to the consideration of the teeth, which I have purposely reserved to the 
last. With a general agreement in number and composition, these important organs 
offer, in the three animals under consideration, differences so essential as to justify of 
themselves, but more especially when considered in connexion with the striking modi- 
fications in the form of the crania and in the organs of locomotion, the formation of a 
distinct genus for the reception of each. In all, the incisor teeth have the number and 
form which are common to nearly the whole of the order: they are two in each jaw, 
chisel-shaped at the apex, and those of the upper jaw have their exserted portion nearly 
vertical, while those of the lower pass obliquely forwards and upwards. The diastematic 
space between them and the molars is considerable ; and the latter are four in number 
on each side of both jaws. They are all constructed nearly upon the same model, 
having no distinct roots, and being each composed of either two or three parallel, 
ribband-like amine of osseous matter, each lamina surrounded by its own proper coat of 
enamel, and united to its fellow by an intervening cortical substance. In Lagostomus 
the lamine are two in number in each tooth, with the exception of the hinder one of 
the upper jaw, which has a third but smaller lamina superadded posteriorly ; and the 
lamine on the worn surfaces of the teeth are perfectly straight, and nearly equal. In 
Lagotis the number of laming in each tooth is increased to three; the teeth of the 
upper jaw have the posterior, and those of the lower the anterior, lamina smaller than 
the others ; and these smaller lamine do not, in the former case, advance to the inner, 
or in the latter case to the outer margin of their respective teeth. In consequence of 
this arrangement the teeth of the lower jaw exhibit an appearance in some degree the 
reverse of those of the upper, and this effect is still further heightened by. the latter 
having its posterior, and the former its anterior, tooth prolonged into a triangular 
shape, while all the rest are square. The two larger Jamine form on the worn surfaces 
of the teeth regular curves with the convexity directed forwards in the upper jaw and 
backwards in the lower ; and the crowns exhibit in the former two grooves externally 
and one internally, marking the line of union of the separate lamelle, while in the latter 
the grooves are, like the lamine themselves, reversed. In Chinchilla the number of 
VOL. I. I 
