After failing for a full day to make sense of an 1888 shell edging pattern, I found a different shell edging in the same book: Weldon’s Practical Needlework, vol. 1, p.14. The second edging had only 2 small errors in it that are now corrected. I also changed the diamonds from garter stitch to stockinette stitch to help distinguish them from the shells.
The edging is 3 inches at its widest at gauge.

Skills needed: cast on, slip, knit, purl, YO, k2tog, SK2P and bind off in knit stitch.

Gauge is not important in this pattern. Any block-able yarn/thread will work. Make a swatch to check gauge and appearance.

45 yards of Aunt Lydia’s Classic 10 thread will make 10 inches of this edging at gauge.