26 ECHINOIDEA. 



are seldom preserved in fossil species ; they are admirably shown, however, in the fine 

 specimen of Cidaris subvesiculosa (PI. VIII, fig. 4), from the collection of our kind 

 friend, Dr. Bowerbank, F.R.S. 



The Tubercles. 



The plates composing the test of the Cidaridoe, Echinida, and Salenida, have 

 large tubercles developed on their surface; they are divided into primary tubercles, semi- 

 tubercles, secondary tubercles, minute tubercles, granules, and miliary granulation. In the 

 other families the tubercles are smaller, more numerous, and less complicated. 



The primary tubercles form two rows in the inter-ambulacral areas of the Cidarida, 

 (PI. I and II), and four, six, eight, ten, or twelve rows in many Echinida. 



The semi-tubercles are found at the base of the ambulacral areas of the genus Hemi- 

 cidaris. In a section of the genus Hemipedina they likewise are found at the base of the 

 ambulacra. 



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

 ambulacral side of the primary tubercles. They are found likewise on the centro-sutural 

 side of the primary rows of many Pseudodiadema, Pedina, Hemipedina, and Echinus. 



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

 areas of the genus Cidaris (PI. VIII, fig. 1) ; and they sometimes fill up spaces at the base 

 of the inter-ambulacral areas. In some Pseudodiademata they are raised on little 

 eminences, and 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 

 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, or form circles around the 

 circumference of the areolas of the primary tubercles (PI. VIII). 



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

 in the centre of the ambulacra, or on the inter-tubercular surface of the large plates 

 composing the inter-ambulacra (PI. VIII, fig. 1). 



The primary tubercles of the genera Cidaris, Hemicidaris, Pseudodiadema, 

 Hemipedina, Pedina, &c, consist of the following parts : 



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

 times imperforate, as in many Echinida (PI. VII, fig. 3 c). 



The boss, or mammillary eminence, is the conical prominence on the surface of the 

 tubercular plate; its summit supports the tubercle, and the margin thereof is crenulated 

 in Hemicidaris and Pseudodiadema, and smooth in Hemipedina and Pedina. 



These characters are important for generic distinctions. 



