Defects of the caudal end of the vertebral spine are not so uncommon as one would imagine if judging from the literature. This deformity has been considered and reported more frequently in recent years, but it has had scanty mention when compared with the frequent reports of deformities in other parts of the spine, for instance, the Klippel-Feil syndrome. A plausible explanation is that deformities of the caudal portion of the vertebral spine are usually combined with extensive involvement of the viscera and the lower extremities and are not compatible with life and that hence they are of more theoretical anatomic interest. Many of the peculiar sirenes belong to this group. As a rule, these conditions show, in addition to the malformation of the lower extremities, considerable numerical reduction of vertebral segments, either complete, involving vertebral bodies and arches, or incomplete, with partial development of some of the segments. However,