1252 CLASS MAMMALIA. 



Finally, the sclerotic of the eyeball never develops a ring of bony 

 plates like that so frequently found in the Sauropsida. 



A characteristic feature of the larger bones of Mammals, although 

 one by no means peculiar to the class, consists in their ossifying 

 from several distinct centres. In the case of the long bones the 

 shaft is formed by one centre of ossification, while two distinct ele- 

 ments termed epiphyses form the extremities ; in the adult the whole 

 of these being welded together into a solid mass. The long bones 

 are also tubular, and their vacuity is filled with the fatty marrow. 



The vertebras always have well-developed articular processes on 

 their arches ; the ends of the centra are generally flattened, but in 

 the cervical region of certain Ungulata they may be opisthoccelous. 

 Terminal epiphyses, so generally wanting in the Sauropsida, are 

 nearly always present. The number of vertebrae varies greatly, 

 owing to the great difference in the length of the tail in different 

 species ; but in the majority of Mammals the number of precaudal 

 vertebrae does not vary very far from thirty, although in Hyrax 

 and Cholcepus their number reaches forty. In spite of the great 

 difference in the length of the neck in different Mammals, the 

 number of cervical vertebrae in existing forms is, with three normal 

 exceptions, seven. These exceptions are Manatus australis and 

 Choloepus Hoffmanm, in which the number is reduced to six, and 

 Bradypus tridactylus, in which it is increased to nine. Accord- 

 ing, however, to Professor W. K. Parker there may occasionally 

 be eight cervicals in the Pangolin (Mam's). The first, or atlas, 

 vertebra always has two articular cups for the occipital condyles; 

 and, except in certain Cetacea, the second, or axis, has a well-de- 

 fined odontoid process. Usually the cervical vertebrae are quite 

 free ; but they are anchylosed together in some of the Cetacea, and 

 in the Armadillos. The dorsal vertebrae are usually well defined 

 from the lumbar, although this is not invariably the case ; and the 

 number of dorso-lumbars in any one given group is usually very 

 constant, and among the Ungulata affords assistance in classification. 

 A distinct sacral region is present in all Mammals except the Ce- 

 tacea, where the iliac bones are absent. The number of caudal 

 vertebrae varies from three (certain Primates) to forty-six (Mam's). 

 Chevron-bones are present in the caudal region of many long-tailed 

 forms. The sternum is always present, although varying greatly in 

 form. It usually consists of a presternum (fig. 1 1 2 8, p) and of a 

 posterior xiphisternum (x), between which are a varying number of 

 segments (m) constituting the mesosternum. The segments of the 

 mesosternum (fig. 11 28, a) may be anchylosed together; and in 

 the Balcenidce, among the Cetacea, only the presternum is present. 

 The connection of the ribs with the sternum is generally by carti- 

 lage ; but in the Armadillos the costal cartilages ossify, and are then 



