IlODENTS. 
55 
Leopard-Sea], Ogmorldnus leptonyx ( 646 ), Ross’s Seal, Ommato- 
plioca rossi ( 647 ), and the Orab-eating Seal, Lohodon carcino- 
pJiayus ( 649 ). Very different is the gigantic Sea-Elephant or 
Elephant-Seal, Morirnf/a, or Macrorhimis, leonina ( 650 , fig. 31), 
the largest member o£ the family, and likewise a southern type. 
The large case in the middle line of the Gallery contains a 
unique series of the local races of this species. Several of tlie 
specimens were presented by the Hon. Walter Rothschild. 
Order RODENT I A, or Gnawing Mammals. 
(Upper Gallery, Cases 10, 11, 11 A, 11 B, 12.) 
The Rodentia, or Gnawing Mammals, comprise Squirrels, 
Rats, Hares, &c., and form by far the largest order of Mammals, 
containing over 2000 distinct species. As a whole. Rodents 
are distinguished by their small size, nocturnal habits, and 
vegetarian diet, all of them living mainly on fruits, leaves, nuts, 
and other similar food, although many species will occasionally 
eat eggs, birds, fish, or other animal food. They are especially 
characterized by their want of canine teeth and by the peculiar 
structure and great development of their incisor, or front, teeth. 
The majority have only a single pair of incisors above and 
below, which are large, curved, and adapted for gnawing by 
possessing sharp, chisel-like edges, formed by the hard outer 
coat of enamel, restricted to the front surface, and wearing av/ay 
more slowly than the softer dentine or tooth-core. These teetlo 
moreover, continue to grow during the whole life of the animal 
from their roots as fast as they wear down at their tips. Should, 
however, one of them get destroyed or diseased, the corre- 
sponding tooth in the opposite jaw, which ought to have been 
worn down by it, continues to grow until it may even bring 
about the death of the animal by preventing the mouth from 
closing, and thus causing starvation, or by curving right over 
and entering the back of the head. Examples of such abnormal 
developments are exhibited in the North Hall. The collar-bones, 
or clavicles, vary in their development, being, as is usual 
throughout the Mammalia, complete from end to end in those 
