GENERAL CHARACTERS OF MAMMALS. 635 



of Monotremes and Sirenia. The centra of the vertebra have 

 generally flat or slightly rounded faces, and there are usually 

 seven cervical vertebra. 1 



The bones of the skull are finnly united by sutures, which 

 generally persist. Only the lower jaw, the ear ossicles, and 

 the hyoid are movable. There are two occipital condyles, as in 

 Amphibians. It may be noted, however, that for various 

 reasons, e.g. that some Birds and Reptiles are not very 

 clearly single-condyled, morphologists no longer attach so much 

 importance to this character as they once did. The lower jaw 

 on each side consists, in adult life, of a single bone which 

 works on the squamosal ; the quadrate which intervenes in 

 Sauropsida has disappeared, or has been shunted to become 

 one of the ear ossicles. For it is a plausible theory of the 

 three ossicles — 7nalleus, incus, and stapes — which connect the 

 drum with the inner ear, that they correspond respectively to 

 the articular, quadrate, and columella or hyo-mandibular of 

 other Vertebrates? The otic bones fuse with each other to 

 form a compact periotic. A bony palate, formed from pre- 

 maxilla, maxilla, and palatines, separates the buccal cavity 

 from the nasal passages. In most cases there are teeth, borne 

 in sockets by the premaxilla, maxilla, and mandible. 



Except in Monotremes, the coracoid is represented by a 

 small process from the scapula, and sornetimes by a small 

 ossification, forming part of the glenoid cavity in which the 

 head of the humerus works. The sternum includes — (a) a 

 prasternum, to which in Monotremes an " interclavicle " is 

 fixed, and with which the clavicles {if well developed) 

 articulate; (b) a mesosternum divided into segments, with 

 which the sternal parts of the ribs articulate ; and (c) a 

 xiphisternum, often cartilaginous. There are generally two 

 sacral vertebra, but several caudals, and more rarely a 

 lumbar, may be fused to these. The ilio-sacral articulation 



1 In the Manatee there are, however, only six ; the pangolin Manis 

 has sometimes eight ; and it is often said that the two-toed sloth 

 (Chokepis hoffmanni) has only six, and the three-toed sloth {Bradypus 

 tridactylus) nine ; but in the case of the sloths there is apparently con - 

 siderable variation. It will be noticed that these deviations from type 

 occur only in the case of the two most old-fashioned orders of Eutherian 

 Mammals. 



2 There are many other theories as to the quadrate, that it forms 

 malleus, or tympanic ring, or zygomatic process of squamosal, etc. etc. 



