MAMMALIA. 361 



stitute the hard palate and separate the narial and buccal cavities. There 

 are distinct lacrymal and tympanic bones, and the latter is often dilated 

 into a tympanic bulla and prolonged outwards as an external bony meatus. 

 The periotic bones anchylose inter se, and form a compact periotic mass 

 which is either connected by suture to neighbouring bones or is partially 

 or wholly free. The carotids enter the cranial cavity, either by piercing 

 the periotic mass or passing between it and the base of the skull. The 

 rami of the lower jaw articulate directly with the squamosal bones. Each 

 ramus consists in the adult of a single bone which unites with its fellow at 

 the mental symphysis either by suture or by anchylosis. It is derived in 

 Man from four centres of ossification which correspond respectively to a 

 mento-meckelian, dentary, splenial and coronoid element. There are three 

 auditory ossicles, a malleus, incus and stapes, which represent, the first- 

 named, the articular element of the lower jaw, the second, the quadrate 

 bone, and the third the columella auris of the Sauropsida and the hyoman- 

 dibular of Fish. The cervical vertebrae are reduced to six in the Manatee, 

 in Choloepus Hoffmanni among Edentata, and increased to nine in Bradypus 

 tridactylus in the Order named. Cervical ribs are represented only by 

 centres of ossification often absent. The rib-element is often wanting in 

 the seventh cervical vertebra ; it is sometimes present abnormally, however, 

 as a free rib. The cervical is always sharply marked off from the dorso- 

 lumbar series of vertebrae. The latter series varies in number between the 

 extremes of 14 in the Armadillo (Edentata) and 30 in Hyrax. The number 

 is often constant within the limits of a given group, e. g. to 19 in Artio- 

 dactyla among Ungulata. The dorsals are usually 12 or 13. There may 

 be but a single sacral vertebra as in Perameles among Metatheria, or more 

 commonly two ; and the number is generally increased by the anchylosis 

 of a variable number of anterior caudal vertebrae, more rarely of a pos- 

 terior lumbar. A sacral region is not defined in Cetacea and Sirenia where 

 the ilia either fail to reach the backbone, or else are absent. The caudal 

 series may be reduced to 35 vertebrae in Man and the higher Apes, but 

 is generally numerous. The largest number known, 46, occurs in Manis 

 macrura among Edentata. The two first cervical vertebrae articulate one 

 with the other, and with the skull by synovial joints : the remaining verte- 

 brae by fibro-cartilaginous discs, in the axis of which is found a remnant 

 of the notochord, the nucleus pulposus. The centra of the vertebrae are 

 usually flat, but in the Ungulata those of the cervical region in particular 

 are more or less opisthocoelous. The ribs are divided into a vertebral and 

 sternal section, the latter sometimes cartilaginous, sometimes ossified. 

 Some of the posterior ribs may lose their connection with the sternum, 

 and also with the vertebrae, and in the latter case are known as floating 

 ribs, e. g. in Cetacea. The sternum undergoes, at a certain stage of growth, 

 transverse segmentation into a series of sternebrae which may or may not 



