146 
number produced in the upper jaw (though they are not all present 
at any one stage) is about 20 on each side. These posterior alternating 
teeth are much larger in size than the anterior, and while they are 
being formed the latter are undergoing displacement by vertical suc- 
cessors (Figs. 2 and 3). It will be noted that in the upper jaw 
the behaviour ofthe alternating series differs greatly 
from that ofthe same series in the lower jaw, for whereas 
in the former the teeth enlarge in size and separate from one an- 
other for many months after hatching, those of the latter cease growth 
OK + > ; yiVV U 
Ye CIYV MM ZV IW | V V 
a vg Ä \4 eg H ¥ 
pmz N ) ji IN i N 
N N A Artes 
mb ME Be 
Fig. 1. The marginal teeth of an embryo in the last month of incubation. a) Side 
view; b) the anterior teeth. Man mandible. pmax premaxilla. Length of skull 
13,5) mam. eo: 
Fig. 2. Semi-diagrammatic figures of the marginal teeth, a) in a specimen 17 em 
long (a few months after hatching), b) in a specimen 21,2 cm in length. JJ, III, IV 
indicate the several dentitions to which the teeth belong. U.S. uniform series. The 
dotted line divides premaxilla from maxilla. X about 2!/,. 
when the animal is a month or two old and occupy practically the 
same length of jaw up to the time they are worn away, or in the 
case of the anterior teeth, displaced by their vertical successors. The 
cause of this difference is to be found in the fusion of the teeth 
which takes place in the lower jaw just before hatching (Fig. 5), and 
