MAMMALIA. 363 



medulla oblongata and spinal cord is generally considerable. The brain 

 of the gigantic extinct Mammalia from the American Eocene was remark- 

 ably small. The cast of it passes with ease through the neural canal of 

 the vertebrae, and is Lacertilian in aspect, with large olfactory and small 

 cerebral lobes. The sympathetic cord accompanies the vagus nerve in the 

 neck, and has never more than three cervical ganglia. There are no 

 sensory hairs to the cells of the olfactory epithelium. The eye is rudi- 

 mentary in Platanista among Cetacea : rudimentary or absent in certain 

 burrowing Rodentia and Insectivora. There is often a tapetum lucidum 

 external to the retina, composed of fine parallel waved connective tissue 

 fibres as in Ungulata, or of cells as in Carnivora. There is, except in 

 Primates, a suspensory or choanoid muscle to the eye internal to the circle 

 of recti muscles ; and in Carnivora it is broken up into four muscles. The 

 nictitating membrane is present except in Primates and Cetacea, but has no 

 special muscle. The eyelids are reduced in Sirenia to a circular fold which 

 contracts to a point. The Cetacea have no lacrymal glands. In the inner 

 ear the cochlea is spirally convoluted. The external aperture of the ear is 

 a simple aperture in most aquatic and burrowing Mammals, but in others 

 it has a well-developed and characteristic pinna. For the ear-bones, see 

 p. 361. 



A few Mammals are edentulous as the Ant-eaters (Myrmecophaga, 

 Cyclothurus, and Manis] ; or there may be transitory teeth which never cut 

 the gum (Baleen Whales). Teeth are limited to the prae-maxillary, 

 maxillary, and mandibular bones. They are implanted in sockets by one 

 or more roots, and are attached to the socket by a fibrous alveolo-dental 

 membrane, and anchylosis is said to occur only with the incisors of certain 

 Shrews (Insectivora]. The teeth are either simple and alike in shape 

 (homodont), or differ from one another (heterodont), and are then termed 

 in a complete series incisors, canines, praemolars and molars. When their 

 number is limited, as is usually the case, the full dentition includes 44 teeth, 

 that is to say n teeth on each side above and below, viz. 3 incisors, I 

 canine, 4 praemolars, and 3 molars, as in the Pig among Ungulata, and 

 the genera Gymnura, Myogale and the Mole (Talpa) among Insectivora. 

 But in living Mammals the number generally falls below this maximum. 

 In homodont dentitions, however, the number is often great, e. g. 100 in 

 Priodon among Edentata, 200 in Delphinus among Cetacea. There are two 

 sets of teeth, a milk and a permanent, in the majority of Mammals, hence 

 termed Diphyodonts, as opposed to Monophyodonts, which have but one 

 set. It is probable that the single set of teeth of Monophyodonts corres- 

 ponds to the permanent set in Diphyodonts. The milk dentition includes 

 incisors, canines, and deciduous (prae)molars which are replaced by corre- 

 sponding teeth in the permanent set as a whole or in part. The milk 

 dentition is sometimes shed in tttero (Guinea-pig), absorbed at an early 



