different between librigena vs fixigena

English

librigena (plural librigenae)

(anatomy) The "free cheek" cephalic shell on either side of the glabella outside of the facial sutures.

http://www.fossilmuseum.net/glossaries/trilobitesglossary.htm

fixigena (plural fixigenae)

(anatomy) The "fixed cheek" cephalic shell surrounding glabella inside of facial sutures.

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