The Collaborative International Dictionary
Peritomous
Peritomous \Pe*rit"o*mous\, a. [Gr. ? cut off all around. See Peri-, and Tome.] (Min.) Cleaving in more directions than one, parallel to the axis.
Wiktionary
peritomous
a. (context mineralogy English) cleave in more than one direction, parallel to the axis.