542 CHORD AT A. 



A 



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, N'otoryctes. 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. 



VIII. — The Porpoise {Fhocce?ia communis). — Natatorial 

 OR Aquatic. 



The porpoise belongs to the order Cetacea (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, i,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 



