The Classification of the Clypeastroidea adopted by Agassiz is based chiefly on the degree of development of the internal skeleton, and as this is of great physiological importance to the animals we shall follow it here; but since it was published the remarkable discovery has been made of Pygastrides, a type previously known only from fossils. We must therefore recognise two sub-orders:—

Sub-Order I. Protoclypeastroidea.

Anus on dorsal surface near apical pole. One species, Pygastrides relictus,[[501]] with no "petals," from deep water in the Caribbean Sea.

Sub-Order II. Euclypeastroidea.

Anus on under surface.

Fam. 1. Fibularidae.—The "petals" are short and imperfect, and the internal skeleton consists of two short outwardly-directed septa in each interradius. To this family the only British Clypeastroid, Echinocyamus pusillus, belongs. This animal never exceeds an inch in length, and has an oval outline. It inhabits shallow water, and is often found in the same ground as Echinus miliaris, but like all Clypeastroids it prefers a sandy bottom.

Fam. 2. Echinanthidae or Clypeastridae.—"Petals" well marked, internal skeleton consisting of isolated pillars. The largest Cake-urchins belong to this family, which is found chiefly in tropical waters. Clypeaster, the great Cake-urchin, with a deeply sunken peristome, belongs to this family.

Fam. 3. Laganidae.—Closely allied to the foregoing, but distinguished by the fact that the internal skeleton consists of walls parallel to the edge of the test. (Laganum, Arachnoides, Peronella.)

Fam. 4. Scutellidae.—This family includes about half the genera, and is sharply distinguished from all the rest by (1) the extremely flattened shape, (2) the indentation of the outline in the anal interradius and often elsewhere, (3) the branching of ambulacral furrows on the under surface. Echinarachnius, taken as the type in describing the anatomy of the Cake-urchins, is the best-known genus. Others are Mellita, with five perforations in the edge of the test; and Rotula, with the edge produced into a number of finger-like processes.

Order III. Spatangoidea (Heart-urchins).