Round River Farm... 

...grows an heirloom variety called Hartley. In the 20th century, it became the #1 planted variety of walnut grown in California, easily over half the acreage of all commercially produced walnuts. It was popular because it was considered easy to crack, large, light colored, and delicious. As an orchard tree, it was productive and grew vigorously. But for reasons my grandmother would never have understood, it "fell out of favor" in the marketplace. For the same reason I will never understand why Whole Foods thought of selling peeled oranges in a plastic clamshell, she couldn't imagine that a lot of people find cracking and shelling a nut to be a chore (she used a brick and a hammer on the kitchen countertop). So the industry moved into the shelling business in the 1970's & '80's, and Hartley doesn't really fit the machines that were invented. New varieties with a more spherical shape worked very well in the mechanical crackers, and resulted in beautiful, complete 'halves'. And that's what you find, mostly, in the grocery stores today.