These joints (peculiar to equidæ) are diarthroses formed by the
transverse processes of the last two lumbar vertebræ and the alæ of the
sacrum. A similar joint between the fourth and fifth lumbar processes is
frequently present. The =articular surfaces= have an elongated oval
form, the anterior one being concave and the posterior one convex. The
capsule is tight, and is reinforced ventrally.