ECHINODERMA. ECHINOIDEA 121 



With only a few exceptions the corona in the Mesozoic 

 and later echinoids is formed of twenty columns of plates, 

 as described above ; but in the Palaeozoic echinoids, more 

 than twenty columns of plates are found (fig. 51), except 

 in the earliest-known genus Bothriocidaris (Ordovician), 

 which is remarkable in having only one column of plates 

 in each interambulacral area, with the usual two columns 

 in each ambulacral area. In other Palaeozoic forms the 

 number of columns is variable and often great, so that the 

 total number of plates in the corona becomes considerable : 

 thus, Archceocidaris possesses two columns in each am- 

 bulacrum, and from four to seven in each interambulacrum ; 

 Oligoporus has four ambulacral and from four to nine 

 interambulacral rows ; Melonechinus, five to fourteen am- 

 bulacral, and from four to eleven interambulacral columns ; 

 whilst Lepidesthes consists of from eight to as many as 

 eighteen ambulacral, and three to six interambulacral 

 columns. In these Palaeozoic forms each ambulacral plate 

 possesses one pair of pores. 



In most echinoids the test is rigid, but in some the 

 plates of the corona overlap to a slight extent, giving some 

 flexibility to the test ; such is the case in several Palaeozoic 

 genera, and also in a few later forms, viz. Pelanechinus from 

 the Corallian, Echinothuria from the Chalk, and some 

 living species of the deep-sea Asthenosoma and Phormo- 

 soma. 



The plates of both the ambulacral and interambulacral 

 areas are often provided with rounded elevations known 

 as tubercles and granules. The tubercles are of various 

 sizes, the largest being the primary. In these the follow- 

 ing parts may be distinguished : at the summit a hemi- 

 spheroidal piece, sometimes perforated at the top, and 

 known as the mamelon (fig. 49 B, m). The mamelon rests 



