大毘盧遮那成佛經疏 14.13

14.13

次舒竪左手，令指頭勿相著，即以空地相捻，捻小指甲也，是商羯羅印也（是骨璅天也）。如前印而並合三指。即是烏忙那印。是商羯羅之后也。二印相似。前頭指少屈。此直竪而散也。左手作半蓮華印。一如作觀音印法。此不同。但一手作之耳。此是梵天印。

Next, extend and erect the left hand, keeping the fingertips from touching one another, and pinch space and earth together, pressing the nail of the little finger; this is the Śaṅkara mudrā (he is the Bone-Chain Deva). As with the previous mudrā, join the three fingers side by side: that is the Umā mudrā, who is the queen of Śaṅkara. The two mudrās are similar; previously the forefinger was slightly bent, but here it is straight, erect, and separated. Form a half-lotus mudrā with the left hand, just as in the method of forming the Avalokiteśvara mudrā; here it is different only in being performed with one hand. This is the Brahmā Deva mudrā.