Each scale is an approximately circular sheet of extracellular matrix material. An
outer osseous layer covers a more interiorly situated fibrillary plate (Fig. 1). This
plate mainly consists of layers of collagen fibres stacked in an ‘orthogonal cross-ply’
configuration (Fig. 4). Cells that form a layer covering the outer osseous surface of a
scale will be referred to as osteoblasts (Van Oosten, 1957) in this account, and cells in
the layer that is situated against a scale’s inner fibrillary surface will be called
fibroblasts (Onozato & Watabe, 1979) (Fig. 1). Osteoblasts and fibroblasts have also
been termed scleroblasts (e.g. see Byers & Fujiwara, 1982) or described as
episquamal and hyposquamal cells (Waterman, 1970), respectively, by previous
investigators. Several layers of epidermal cells cover the osteoblast layer, and layers
of dermal cells and pigment cells are situated inside and against the fibroblast layer
(Waterman, 1970). They have not been examined in detail in this study. The osseous
layer of each scale exhibits a polarized pattern of ridges (circuli) and grooves (radii)
on its outer surface that is oriented with respect to the anterior/posterior axis of the
organism (Fig. 2).