1 66 AN INTRODUCTION TO THE STUDY OF FOSSILS 



masticating apparatus, the Aristotle's lantern. Madreporite 

 dorsal, in the right anterior genital plate ; this plate orients 

 the sea urchin, the antero-posterior axis passing through the 

 ambulacrum on its left and through the opposite interambula- 

 crum. The anal opening is dorsal, in the posterior inter- 

 ambulacrum. 



Straight or variously arranged ambulacral areas run from the 

 dorsal to the ventral surface. 



Movement is by means of the elongated tube-feet which 

 pass through, not between, the plates. Many forms in addi- 

 tion move by means of their spines, often quite rapidly. 



Sea urchins are gregarious, frequently occurring in such abun- 

 dance as to pave the surface of the rock and the bottom of tide 

 pools in sheltered places. On exposed coasts, many forms bore 

 cavities by their teeth or by absorption into calcareous and non- 

 calcareous rocks. Common examples are Strongylocentrotus, 

 on the Atlantic coast north of Cape Cod; Arbacia, on the Atlan- 

 tic coast south to Florida; Echinarachnius, the " sand dollar," 

 on sand beaches from New Jersey northward and on the Pacific 

 coast. 



Echinoids are known from the Ordovician to the present. 



Derivation of name. — Greek echinos, a hedge-hog, + eidos 

 (oid), form, referring to the spines so characteristic of all members 

 of this class. 

 Strongylocentrotus (Figs. 66, 67). Upper Tertiary to present. 



This differs from the starfish in its globular form, w^hich is 

 slightly compressed dorso-ventrally. The calcareous plates 

 are firmly fitted together like a mosaic, forming a rigid shell. 

 The entire surface, except the region around the mouth, is thickly 

 studded with many rounded tubercles, to which are attached 

 solid spines which are movable by muscles on ball and socket 

 joints (Fig. Oy, B). Corresponding to the five ambulacral grooves 

 of the starfish are five slightly expanding ambulacral areas (am- 

 bulacra) extending radially from near the center of the dorsal to 

 the ventral side. Each ambulacrum consists of two narrow 



