Fig. 5

Harriotta raleighana (Rhinochimeridae; Holocephali; Chondrichthyes). Microstructure of rostral lingual hypermineralized outer layer using different optical methods to illustrate tissue composition. a, b micrographs in reflected incident light of hard tissue section from the most rostral end of lower dental plate show position of mineral-dense outer layer on lingual surface (rol, l.os), that is outside all other tissues (field of B in A; field of C, D in A), less hypermineralized than oral ovoids, similar amount to that of sclerotic dentine (sod). Stack of ovoids at oral surface (or.ov) with forming one below, aboral (fov). c transmitted light, Nomarsky optics of outer layer, with masses of fine lines crossing each other, may reflect grouping of crystals normal to surface, or fine tubules of mineralizing trioral dentine (see Fig. 6e, f). d same in polarized light with gypsum plate (−ve 45°) to show organisation of mineral content, blue suggests normal position of hydroxyapatite crystals, none visible in C, but brown fine lines. Lines in A may be incremental growth from the outer surface (icl)