14 TERMINOLOGY. 



The secondary tubercles are found in many of the Echinida, ranged in general on the 

 ambulacra! side in the primary tubercles. They are found likewise on the centro-sutural 

 side of the primary rows (PL VII, fig. 4, and PL VIII, fig. 1) of many Diadema, Pedina, 

 Hemipedina, and Echinus. 



The minute tubercles are the small tubercles which are found on the margins of the 

 ambulacral areas in the genera Cidaris and Hemicidaris. (PL III, fig. 1 b, c; PL IV, fig. 1 d.) 

 They sometimes fill up spaces at the base of the inter-ambulacral areas, as in some Diademas ,• 

 (PL VI, fig. 2y) they are always raised on little eminences, and are in general perforated 

 like the secondary and primary tubercles. In this respect they are distinguished from the 

 granules. 



The granules are small, round, hemispherical elevations, scattered more or less regularly, 

 and distributed over different parts of the plates of the test. In some species of the genus 

 Cidaris, they are arranged in rows in the centre of the ambulacral areas (PL 1, fig. 4<e; 

 PL II, figs. 1 c, 2 y ; PL III, fig. 2 d); or they form circles around the circumference of 

 the areolas of the primary tubercles. (PL II, fig. 2 jr.) 



The miliary granulation is formed by a number of small granules closely set toge- 

 ther in the centre of the ambulacra (PL III, fig. 6 a : PL V, fig. 2 d), or on the inter- 

 tubercular surface of the large plates forming the inter-ambulacra. (PL II, fig. b, c ; PL VI, 

 fig. 2 c, d.) 



The primary tubercles of the genera Cidaris, Hemicidaris, Diadema, Hemipedina, 

 Pedina, &c, consist of the following parts : 



The hemispherical tubercle or mamelon, which is sometimes perforated in the centre, 

 as in all the Cidaridce (PL II, fig. 1 c, d) : sometimes it is imperforate, as in many 

 Echinida (PL XVI). 



The boss or mamillary eminence is the conical prominence which rises from the surface 

 of the tubercular plate (PL II, fig. 1 d) ; its summit supports the tubercle ; its margin is 

 sometimes crenulated in Cidaris, Hemicidaris, and Diadema, and sometimes smooth in 

 Hemipedina and Pedina. These characters are important for generic distinctions. 



The areola or scrobicule (PL II, fig. 1 c) is the round, oval, or elliptical, smooth, 

 excavated space which surrounds the base of the boss. This space is sometimes wide 

 (PL II, fig. 1 c), sometimes narrow (PL IV, fig. 1 d) ; sometimes its margin is elevated into 

 a ridge which encircles it completely (PL II, fig. 2 y) ; the scrobicular or areolar circle 

 is then said to be complete ; sometimes the upper and under sides of the circle are want- 

 ing, when it is incomplete, and the areolas in this case are said to be confluent. (PL I, 

 fig. lb; PL IV, figs. 1 d, 2 c.) 



The row of granules, which encircles the areola, is called the areolar or scrobicular 

 circle. 



The miliary zone is the space comprised between the two ranges of primary tubercles. 

 This zone is sometimes destitute of miliary granules, when it is said to be naked ; some- 

 times it is covered with a close-set granulation. We have, therefore, the ambulacral miliary 



