(336 TERMINOLOGY. 



The tubercles attached to the plates composing the test are divided, ac- 

 cording to their importance, into primary tubercles, secondary tubercles 

 miliary tubercles, and granules. We distinguish in the primary tubercles a 

 mamelon which maybe perforate (PL 11. f. /). or imperforate (PL III. f. u) ; 

 the boss is the conical or mammiUary eminence supporting the mamelon, which 

 may be either smooth (PL III./, j), or crenulated (PI. IP./, a; PLIII.f. 11). 



The areola or scrobicule is the smooth circular or elliptical space from 

 which the boss rises (PI. III./. ■;). 



The scrobicular circle is the raised ring of small tubercles enclosing this 

 areola (PL 111./. .;). 



Miliary zones are spaces between any two rows of primary tubercles, either 

 in the ambulacral or interambulacral region, which may or may not be cov- 

 ered by small granules ( PI. 111. /'. .;). 



Sutural impressions are the smooth zones or furrows which mark the junc- 

 tion of adjoining coronal plates (PL VIII. f. 22, 26). 



Angular or sutural pores are found in the horizontal and median ambu- 

 lacra! or interambulacral lines of junction of coronal plates. Radioles or 

 spines ( Pis. I'.. I'.) are the appendages articulating upon the tubercles. The 

 different parts of the spines are the socket by which it is articulated to the 

 tubercles; the lower part of the radiole is called the head, and is sep- 

 arated from the neck, which is usually smooth or finely striated, by the 

 milled ring, — a prominent ridge more or less deeply grooved, serving as an 

 attachment lor the muscles which are to move the spine: beyond the neck 

 we have the body or shaft of the spine. 



The principal parts of the jaws (PL II"., PL XI". ) are the five pyramids 

 in which the teeth proper move. The auricles (PL IP. f. 5, w) are the 

 limestone pieces of the interior of the test, intended for the support of the 

 whole dental apparatus (lantern of Aristotle). 



Fascioles or semitse — apparently smooth bands, but really covered by 

 minute modified spines — are found only in Spatangoids, and will be de- 

 scribed in their appropriate places. 



In Clypeastroids and Spatangoids, owing to the presence of a well-defined 

 longitudinal axis, we distinguish a bivium and a trivium. The bivium is 

 made up of the two posterior lateral ambulacra with the enclosed interam- 

 bulacral space. The trivium is the combination of the three anterior ambu- 

 lacra. We also distinguish anterior and posterior lateral ambulacra, an odd 

 anterior ambulacrum, and an odd posterior interambulacruin. 



Pedicellarise are modified spines (Pis. XXIV. -XXVI). 



