From the article:
Of the 220 folios from 72 pocket bibles sampled, 68% were calfskin, 26% were goat, and 6% were sheep.
Uterine vellum has long been used for the finest works; as the unborn calf has not been exposed to the world it has no scars, welts, or other imperfections that arise from exposure to the environment.
Similarly, the leather used to upholster the Bugatti Veyron supercar comes from cows from a select Austrian alpine herd that is pastured at an altitude that generally keeps them free of mosquitoes and other biting insects, and their pastures have no barbed wire enclosures. It all has a direct impact on the quality of the skin.