One is tempted to wonder how far the habit which certain Lemurs have of carrying their young across the abdomen with the tail wrapped round the body of the mother is a reminiscence of a marsupial pouch. SKELETON. The skeleton of the Mammalia consists almost solely of the endoskeleton. It is only among the Edentata that an exoskeleton of bony plates in the skin is met with. As in other Vertebrates, the skeleton is divisible into an axial portion, the skull and vertebral column, and an appendicular skeleton, that of the limbs. The bones of mammals are well ossified, and in the adult there are but few and small tracts of cartilage left. VERTEBRAL COLUMN.--The vertebral column of the mammals, like that of the higher Vertebrata, consists of a number of separate and fully-ossified vertebrae. The constitution of a vertebra upon which all the usual processes are marked is as follows:--There is first of all the body or centrum of the vertebra, a massive piece of bone shaped like a disc or a cylinder. The centra of contiguous vertebrae {20} are separated by a certain amount of fibrous tissue forming the intervertebral disc, and the apposed surfaces of the centra are as a rule nearly flat. In this last feature, and in the important fact that the centra are ossified from three distinct centres, the anterior and posterior pieces ("epiphyses") remaining distinct for a time, even for a long time (as in the Whales), the centra in the mammals differ from those of reptiles and birds. The epiphyses are not found throughout the vertebral column of the lowly-organised Monotremata, and they do not appear to exist in the Sirenia. [Illustration] [Illustration] FIG. 5.--Anterior surface of Human thoracic vertebra (fourth), x 2/3. _az_, Anterior zygapophysis; _c_, body or centrum; _l_, lamina, and _p_, pedicle, of the neural arch; _nc_, neural canal; _t_, transverse process. (From Flower's _Osteology of the Mammalia_.) FIG. 6.--Side view of first lumbar vertebra of Dog (_Canis familiaris_). x 3/4. _a_, Anapophysis; _az_, anterior zygapophysis; _m_, metapophysis; _pz_, posterior zygapophysis; _s_, spinous process; _t_, transverse process. (From Flower's _Osteology_.) From each side of the centrum on the dorsal side arises a process of bone which meets its fellow in the middle line above, and is from there often prolonged into a spine. A canal is thus formed which lodges the spinal cord. This arch of bone is known as the neural arch, and the dorsal process of the same as the spinous process. The sides of the neural arch bear oval facets, by which successive vertebrae articulate with one another: those situated anteriorly are the anterior zygapophyses, while those on the posterior aspect of the arch are the posterior zygapophyses; these articular facets do not exist in the tail-region of many mammals, _e.g._ Whales.