Lying ventral to the intervertebral disks in many mammals small paired ossicles are occasionally found ; these are called intercentra and are ossifications in the hypochordal bar (see sub section on embryology). They probably represent the places where the chevron bones or hae mal arches would be attached and are the serial homologues of the anterior arch of the atlas (see fig. 1o) ; these intercentra, either as paired or median os sicles, are often found in lizards.
The sacrum consists of true sacral vertebrae, which directly articu late with the sacrum, and false, which are caudal vertebrae fused with the others to form a single bone. There is also reason to believe that vertebrae which are originally lumbar become secondarily included in the sacrum be cause in the development of man the pelvis is at first attached to the thirtieth vertebra, but gradually shifts forward until it reaches the twenty-fifth, twenty-sixth and twenty-seventh ; the twenty-fifth or first sacral vertebra, however, often reverts to the lumbar type and sometimes may do so on one side only.
Taking the vertebrae which fuse together as an arbitrary definition of the sacrum, we find that the number may vary from one in Cercopithecus patas to thirteen in some of the armadillos, and, if the Cetacea are included, seventeen in the bottle-nosed dolphin, Tursiops. Four seems to be about the average in the
mammalia and of these one or two are true sacral. In some of the Edentata the posterior sacral vertebrae are fused with the ischium, in other words the great sacro-sciatic ligament is ossified. The tail vertebrae vary from none at all in the bat Megaderma to forty-nine in the pangolin (Mavis snacrura). The anterior ones are remarkable for usually having chevron bones (shaped like a V) on the ventral surface of the intercentral articulation. These protect the caudal vessels and give attachment to the ventral tail muscles. The ribs in mammals correspond in number to the thoracic vertebrae. In typical pronograde mammals the shape of the ribs differs from that of the higher Primates and man : they are so curved that the dorso-ventral diameter of the thorax is greater than the transverse while in the higher Primates the thorax is broader from side to side than it is dorso-ventrally. In this respect the bats agree with man and the lemurs with the pronograde mammals.