100 ELEMENTS OF COMPARATIVE ZOOLOGY. 



extent and complexity of these convolutions correspond 

 well with the intelligence of the form. 



In the eyes the nictitating membrane or "third eyelid" 

 of the bird is reduced to a small fold at the inner angle of 

 the eye. Except in the whales, and some seals, moles, 

 etc., external ears are developed, while the internal parts of 

 the ear become considerably modified. Thus the quadrate 

 and one other bone pass in to the middle ear, where they, 

 together with a third bone, form a chain to convey sound- 

 waves to the sensory portions. In the inner or sensory 

 portion a spiral outgrowth, the cochlea, occurs (Fig. 64), 

 and in this is a most wonderfully intricate sensory appa- 

 ratus the organ of Corti the functions of which are as 

 yet uncertain. 



The mouth is usually provided with fleshy lips, and all 

 mammals, except some edentates and whales, have teeth. 

 These teeth are always confined to the jaws (cf. Fishes, p. 

 10), being inserted by one or more roots into sockets in the 

 bone. Some mammals have but a single set of teeth 

 throughout life, but the majority have a first or milk den- 

 tition, which is soon lost and replaced by a permanent 

 dentition. Occasionally, as in the sperm-whale, etc., all the 

 teeth are similar in shape, but usually several different 

 kinds occur, the extreme being reached when four types 

 are present incisors, canines, premolars, and molars. 

 The incisors have but a single root, and are found in the 

 premaxillary bone and in the corresponding position in the 

 lower jaw. The first teeth in the maxillary, if single-rooted 

 and pointed, are called canines; and behind these come 

 the molars, with two or more roots. These in turn are 

 subdivided into premolars, which appear in both milk and 

 permanent dentitions, and molars proper, which occur only 

 in the permanent set. The number of teeth and their 



