EOHINOIDEA. 



229 



the tubercles and spines. The tubercles are rounded eleva- 

 tions upon which the spines are carried (figs. 117, A, and 

 121). They vary much in their dimensions, and receive 

 special names, according to their size or position on the test. 

 Ordinarily the tubercle consists of a rounded ball or hemi- 

 sphere (the " mamelon ") supported upon a conical process 

 (the " boss ") which arises from the plate. The ball of the 

 tubercle may or may not be perforated for the insertion of a 

 ligament which is attached to the articular surface of the 

 spine. In many cases (as in fig. 121) the base of the tu- 

 bercle is surrounded by a round or oval, smooth and excavated 

 space which is termed the " areola " or " scrobicule." 



Fig. 121. Hemicidaris crenukiris, showing tubercles, the larger of which are perforated, 

 and are surrounded by an areola. Oolite. 



The spines are movable appendages which are jointed to 

 the tubercles by a sort of " ball-and-socket " or " universal " 

 joint. They are used defensively and in loco- 

 motion, and vary much in length and shape. 

 Sometimes they are very minute ; at other 

 times they attain a length considerably exceed- 

 ing the diameter of the test. Sometimes they 

 are slender, tapering, and truly spine-like (fig. 

 1 1 7, E) ; at other times they are thickened, 

 ovate, or almost globular (fig. 122). The spine 

 fits on to the rounded head of the tubercle by 

 a concave articular surface (" acetabulum "), and 

 there may or may not be a pit at the bottom 

 of this, for the attachment of the ligament before 

 spoken of. Above the acetabulum or socket of the spine 

 there is a prominent ridge or ring, more or less " milled," 



Fig. 1-2-2. Spine 

 of Cidaris glan- 

 diferus. 



