The dorsal surface (Fig. 6) is narrower at its cranial end than is the ventral surface. Its cranial border bears laterally a pair of cranial articular processes (_b_) with their medially directed facets and between them it is concave, so that a large dorsal opening is left into the vertebral canal between the last lumbar vertebra and the sacrum. Caudad of the articular processes are two pairs of tubercles (_f_). These are the fused cranial and caudal articular processes of the sacral vertebræ. Caudad of them are the caudal articular processes of the last sacral vertebra (_d_). Craniolaterad of the middle and cranial tubercles are =dorsal= (posterior) =sacral foramina= (_g_) for the transmission of the dorsal rami of the sacral nerves. Three spinous processes (_a_) appear between these rows of tubercles. They decrease in height caudad. That part of the surface included between the spinous process and the tubercles is made up of the fused laminæ of the sacral vertebræ. That part between the tubercles and a line joining the lateral margins of the dorsal (posterior) sacral foramina is formed by the fused radices of the sacral vertebræ.
The lateral surface may be divided into two parts. Craniad is a large rough triangular area with equal sides and with one of its angles directed ventrocraniad. It is the lateral face of the pseudo-transverse process of the first sacral vertebra (Fig. 6, _c_). A smooth curved surface (the auricular facet) along its ventral edge articulates with the ilium, while the dorsal portion is rough for attachment of ligaments. Caudad is the narrow longitudinal triangular area of the lateral faces of the fused transverse processes of the second and third sacral vertebræ.
=Caudal Vertebræ.= =Vertebræ caudales= (Fig. 1, _y_, and Figs. 8 and 9).--The caudal vertebræ (21-23 in number) decrease gradually in size to the last one. Caudad they become longer and more slender and lose the character of vertebræ. They become finally reduced to mere centra,--slender rods of bone knobbed or enlarged at their two ends (Fig. 8). The last one is more pointed than the others and bears at its caudal end a small separate conical piece, the rudiment of an additional vertebra.