| 
172 The Extinct Rodentia of North America. (February, | 
great number of remains of this genus, I am able to show that it | 
is only in the immature state of the first molar that it exhibitsa | 
double column, and that in the fully adult animal it consists of a | 
single column with a groove on its external face. The dentition ; 
undergoes. other still more important changes with progressing — 
age, so as to present the appearance of difference of species at ai 
ferent periods. T 
The succession of teeth in the Palæolagus haydeni is as fol- 
lows (Fig 20): e 
The earliest dentition of this species known to me is the pres- i 
ence of the two deciduous molars, the first and second in position, | 
before the appearance of any of the permanent series. Each of 
these has two roots, and the crown is composed of three lobes. ) 
In the first, the first lbe is a simple cusp; the two following at 
divided into two cusps each; the second is similar, excepting that 
the simple cusp is at the pittior end of the tooth. The grooves 
separating the lobes descend into the alveolus on the outer sd 
but stop above it on the inner. o 
In the next stage, the third permanent molar is projected, and 
has, like the second deciduous, a posterior simple column, yer : 
section forms an odd cusp or lobe. The fourth true molar thet 
follows, also with an odd fifth lobe behind. This lobed form & | 
the molars is so different from that of the adult as to have led me 
to describe it as indicating peculiar species under the name T 
Tricium avunculus and T. anne. i 
In the next stage, the fifth small molar appears in view, and th r 
second permanent molar lifts its milk-predecessor out of the vay 
In a very short time, the posterior, or odd, columns entirely ‘ dix 
appear, sinking into the shaft, and the permanent molars asst 
the form characteristic of the species. The last stage prior $ a 
of a posterior lobe at this time, and that speedily disappears. — er 
anterior lobe is subconical, and is entirely surrounded with enats 
for a time the tooth presents an 8-shaped section, which was 5H 
posed to be characteristic of the genus. Further protrusion! D 
to the surface the bottom of the groove of the inner side ¢ 
shaft, so that its section remains in adult age something lik¢ 
The oe haydeni was dedicated by Dr. Le a 
