542 CHORDATA. 
2. Transition group: armadillo, aark-vark, bandicoot, rabbit, 
marmot, prairie-dog, &c., show a well-developed burrowing habit 
and a domicile underground, although their food is still in most 
cases obtained terrestrially. The claws are well developed, and in 
many cases the fore-limbs are shortened and the ridges or crests for 
the limb-muscles are prominent. The terrestrial habit is still well 
in evidence, however, and the necessity for speed above ground limits 
the limb-modification. 
3. True fossorial: mole, golden mole, Notoryctes. In this type 
the food is subterranean and the habit is completely fossorial. The 
sense of sight is vestigial, hearing and smell being hypertrophied ; the 
fur is reversible, lying evenly either forwards or backwards, and the 
limbs are essentially fossorial. The body is cylindrical, and the fore- 
limbs are shortened up with powerful keeled sternum and tuberosities, 
digits strong and spreading, with strong claws. In some respects the 
structure resembles that of the swimmers, the motion being somewhat 
similar. Insects and other ‘‘small flesh” form the diet, a truly 
herbivorous fossorial mammal being unknown. 
VIIL—TueE Porpoise (Phocena communis).—NATATORIAL 
OR AQUATIC. 
The porpoise belongs to the order Ce/acea (see page 578) 
and the sub-order Odontoceti (or toothed whales). Its 
general appearance is familiar. It may be anything up to 
five feet in length and is fish-like in shape, 7.e., the body is 
more or less circular in cross-section and is thickest just a 
little anterior to the middle, from which it tapers gradually 
to the tail, more abruptly to the head. It is dark greyish- 
green on the upper half of the body and head, on the tail 
and fins, and a pearly-white on the under surface. The 
surface of the body is smooth and oily and there is no hair. 
The mouth, with wide gape, is at the front end of the head, 
the eyes are lateral and small with no lacrymal glands, whilst 
the external ears are absent. A minute pin-hole leads from 
the exterior to the tympanum on each side, and at the top of 
the head is a single crescentic nostril which is open or closed 
as required. About a quarter of its length from the head 
the paired fins are seen protruding ventro-laterally, formed 
from much modified fore-limbs. Behind the middle line 
here is a median dorsal fin, and the tail is modified into 
a bilateral or symmetrical tail-fin, the “flukes” of which 
lie horizontally. Thus we find that externally in form, 
colour, reduction of ears and loss of hair, the porpoise is 
