Vou. 2] Robertson.—Non-Incrusting Bryozoa. 291 
Flustra Linneus. 
Flustra sp., Linn., 1758. 
Eschara (part) Pallas, 1766. 
Flustra, Smitt, 1867. 
Flustra, Busk, 1852. 
Carbasea Busk, 1852. 
Flustra, Hincks, 1880. 
Zoarium erect or sub-erect. Zowcia disposed in a single or 
double layer, more or less quadrangular or linguiform, with a 
raised margin, the aperture occupying the whole or a consid- 
erable portion of the front, and closed in by a membranous 
covering. Oacia immersed or raised. 
34. Flustra lichenoides Robertson. 
Pi XV, figs. 91, O25-Pi VE, figs. 105. 
Flustra lichenoides Robertson, 1900, p. 322, Pl. XX, figs. 7, 7a, 8. 
Zoarium unilaminar, consisting of broad foliaceous green or 
brownish green fronds. (Pl. XVI, fig. 105.) Zowcia in alter- 
nate rows, slightly arched above, narrowed below, the distal 
margin raised and armed at each corner with an erect stiff 
spine (fig. 91 e. sp.), often slightly curved; below on each side 
of the aperture two other spines (c. sp.), flattened and eurving 
inward, often meeting in the middle above the aperture ; aperture 
occupying the whole front. Oacia (oe.) rather low, but raised 
above the surface of the zocecia; not as high as the erect spine 
at its base. Avicularia? Rootlets (fig. 92 r. f.) arising from 
the upper corner of most of the zocecia by means of which the 
fronds are attached to the substratum, and above which the 
colony does not rise to any height but spreads out in convoluted 
masses. 
Considerable variation occurs in the development of the 
spines. Specimens are found in which no spines are visible 
except those at the corners; others which have but one pair of 
lateral spines, or in which the spines are unevenly developed on 
the two sides. Variation also occurs in the root fibres. These 
frequently anastomose and form a net work of fibres below the 
lowest frond. Root fibres from overlapping fronds often attach 
