Kshurika / Yogashikha Upanishad

Kshurika / Yogashikha Upanishad cited by Swami Vidyaranya

Swami Vidyaranya has cited an Upanishadic passage -

सुषुम्ना तु परे लीना विरजा ब्रह्मरूपिणी -   in the Taittiriya Upanishad commentary.  Upon a search it is seen to be found in these two Upanishads:

Kshurika upanishad:

https://sanskritdocuments.org/doc_upanishhat/kshurika.html  

Yogashikha upanishad:

https://sanskritdocuments.org/doc_upanishhat/yogashikha.html  

These form part of the 108 Upanishads listed in the Muktikopanishad.  Both these Upanishads have the commentary of Narayanashrama/Shankarananda who preceded Swami Vidyaranya.  Of course, the Upanishad Brahma Yogin commentary is also there as part of the 108 Upanishads.  

We come to know through Swami Vidyaranya's writings about the existence of certain Upanishads (in the pre 14th Century CE)  that have not been cited popularly by Shankaracharya and others.