Order v. Discoidea.—Shell flattened, of circular plan, simple or concentric, rarely spiral. (Families 18-23.)
Order vi. Larcoidea.—Shell ellipsoidal, with all three axes unequal or irregular, sometimes becoming spiral. (Families 24-32.)[[87]]
Fig. 24.—Xiphacantha (Acantharia). From the surface. The skeleton only, × 100, (From Wyville Thomson.)
B. Acantharia.
Order vii. Actinelida.—Radial spines numerous, more than 20, usually grouped irregularly. (Families 33-35.) Xiphacantha Haeck.
Order viii. Acanthonida.—Radial spines equal. (Families 36-38.)
Order ix. Sphaerophracta.—Radial spines 20, with a latticed spherical shell, independent of, or formed from the reticulations of the spines. (Families 39-41.) Dorataspis Haeck. (Fig. 25, A).
Order x. Prunophracta.—Radial spines 20, unequal; latticed shell, ellipsoidal, lenticular, or doubly conical. (Families 42-44.)
C. Nassellaria.