Most comparisons between the classical manuals and modern practice turn up discontinuity: names moved to different shapes, and a large standing repertoire with no textual ancestry, a history Mark Singleton sets out in Yoga Body. Shavasana runs the other way.
The posture that closes most contemporary classes is the same shape the Hatha Yoga Pradipika describes, put to the same use the Pradipika gives it, under the same name. Lying flat on the back to remove fatigue at the end of practice is attested in the fifteenth century text and is what happens in a studio today. Among the postures examined in this atlas it is the clearest single case of continuity across the whole span.
That deserves recording as carefully as the discontinuities do. A reference work that only reports where a tradition has changed produces a distorted picture, in which everything modern looks like an invention. The honest account has both: a repertoire whose centre of gravity moved decisively between the medieval manuals and the twentieth century, and inside it particular items, this one and the serpent among them, that travelled with very little alteration. Where a thing has held, the atlas says so, on the same evidence and with the same care it uses when reporting that a thing has not.