ORDER =SIRENIA=. Sea-cows and Dugongs. 21. ORDER =HYRACOIDEA=. Conies. 22. ORDER =†TOXODONTIA=. _a._ SUBORDER =†Toxodonta=. _b._ SUBORDER =†Typotheria=. _c._ SUBORDER =†Entelonychia=. _d._ SUBORDER =†Pyrotheria=. 23. ORDER =†ASTRAPOTHERIA=. 24. ORDER =†LITOPTERNA=. _DD._ COHORT CETACEA. Whales, Dolphins, Porpoises. 25. ORDER =†ZEUGLODONTIA=. 26. ORDER =ODONTOCETI=. Toothed Whales, Dolphins, Porpoises. 27. ORDER =MYSTACOCETI=. Whalebone Whales. CHAPTER IV THE SKELETON AND TEETH OF MAMMALS With very rare exceptions, and those only of the latest geological period (Quaternary), the fossil remains of mammals consist only of bones and teeth. The evolutionary changes, so far as these are preserved, are recorded therefore in terms of dental and skeletal modifications. To render these changes intelligible, it is necessary to give some account of the mammalian skeleton and teeth, with no more use of technical language than is unavoidable; ordinary speech does not furnish a sufficient number of terms, nor are most of these sufficiently precise. With the aid of the figures, the reader may easily gain a knowledge of the skeleton which is quite adequate for the discussion of fossil series, which will follow in the subsequent chapters. I. THE SKELETON I. The most obvious distinction of the skeletal parts is into _axial_ and _appendicular_ portions, the former comprising the skull, backbone or _vertebral column_, ribs and breast-bone or _sternum_, and the latter including the limb-girdles, limbs and feet. In the axial skeleton only the ribs and certain bones of the skull are paired, but in the appendicular all the bones are in pairs, for the right and left sides respectively. [Illustration: FIG. 7.—Skull of Wolf (_Canis occidentalis_). _P.Mx._, premaxillary. _Mx._, maxillary. _Na._, nasal. _L._, lachrymal. _Ma._, malar or jugal. _Fr._, frontal. _Pa._, parietal. _Sq._, squamosal. _Zyg._, zygomatic process of squamosal. _O.S._, orbitosphenoid. _Pl._, palatine. _M._, mandible. _cor._, coronoid process of mandible. _m.c._, condyle of mandible. _ang._, angular process of mandible. _p.g._, postglenoid process of squamosal. _Ty._, tympanic (auditory bulla). _mas._, mastoid. _p.oc._, paroccipital process. _con._, occipital condyle. _Ex.O._, exoccipital. _S.O._, supraoccipital.]