TAST stitch # 97 is called Shell Chain Stitch, and Sharon B of Pintangle, who is running the TAST stitch challenge, says she found it in Edith John's Creative Stitches (p.43).
It is a book I am not familiar with, but the photographs that Sharon has taken of her own stitches and posted on her blog show clearly how this stitch is worked.
Now if I only could pay attention! You see, I've done it again - looked at Sharon's instructions too casually. They made me think of my collection of Totsuka's Embroidery Stitch Books:
where Book No 3 in the series is full of chain stitch variations. Sure enough I found the 'Shell Chain', although it is simply called 'alternating chain and open chain stitch'. (Note also that almost all Totsuka stitches are worked away from you, which I find difficult, and I usually turn the books upside down to follow the illustrations).
I was so upset that I decided to keep them on my sampler just as a good reminder to do a proper job in the future! The Shell Chain Stitch should look like these: