II.—Morphological Studies on the Echinoidea Holectypoida and their Allies

Extract

The conclusions of Lovén as to the composition of the perignathic girdle of D. cylindricus are confirmed in essential particulars, while those of Duncan & Sladen are shown to be based upon imperfectly preserved material. The processes, which may project freely for a varying distance above the rest of the girdle, are slender. The actual ridges are represented, as in Plesiechinus, by the slightly thickened and projecting inner surfaces of the unpaired primordial interambulacral plates. The rest of the interradial portion of the girdle is interpreted as being made of the bevelled edges of the buttresses which radiate over the interambulacra of the adoral surface. These sloping surfaces, here called “false ridges”, are believed to be specialized and somewhat extended as “rests” or “slides” for the inclined pyramids; thus forecasting the articulation between lantern and girdle found in the Clypeastroida. The retractor muscles were attached to the upper parts of the processes, and it is argued that the protractors must have sprung from the small true ridges, the false ridges having no function as muscle supports. In the absence of positive evidence, radial compass muscles are presumed to have been absent.