In a recent paper on “The Unit Cell and Space Group of Probertite” (Am. Mineral.,34, 19-25 (1949)), at the bottom of p. 23 and the top of p. 24, Ftc* and Fta* are shown equal to Fdc* and Fda*, respectively. Since probertite is monoclinic, this is, of course, not true (see bottom of p. 22) when t* and d* have their usual significance. The manuscript of this paper was completed in March 1948 and, at the present date, I am unable to offer any explanation for this error except the possibility of an automatic but irrational reflex in making the b Axis, Zero Level data symmetrical in appearance with those for the a Axis, Zero Level. The aberration does not appear in my original notes. At the bottom of p. 23, c* translation (tc*) and spacing (dc*) should read c* spacing (dc*) and, at the top of columns 2 and 4, Ftc* = should be deleted. Similarly, at the top of p. 24, a* translation (ta*) and spacing (da*) should read a* spacing (da*) and, at the top of columns 2 and 4, Fta* = should be deleted.