234 ANNULOIDA. 



able division of the Echinoidea the test is "regular," the 

 anus being placed in the centre of the apical disc, and the 

 ambulacral areas being continuous ; but the plates of both 

 the ambulacral and interambulacral areas are imbricated 

 and overlap one another (fig. 128), the test thus becoming 

 flexible. In this abnormal character, the Echinothuridce 

 agree with some of the Palaeozoic Urchins, but they differ 

 from these, and agree with the ordinary Eegular Echinoids 

 in having the test composed of no more than twenty rows 

 of plates. 



Fig. 128. Portion of one of the ambulacral areas of Ecldnotliuria floris, enlarged four 

 times. Chalk. (After Wright.) 



The only fossil forms of this group, as yet discovered, 

 are referable to the Cretaceous genus Echinothuria, the true 

 affinities of which have now been elucidated by the dis- 

 covery of the extraordinary living types referred to the 

 genera Calveria (or Asthenosoma) and Phormosoma. 



III. PERISCHOECHINID^E. In this group we have a series 

 of singular Palaeozoic Sea-urchins, which agree with the two 

 preceding sections in having a "regular" test, but which 

 differ from all known Echinoids, living or extinct, in having 

 the test composed of more than twenty rows of plates. The 

 test is still divided into five ambulacral and five interam- 

 bulacral areas, but there is a multiplication of the rows 

 of plates in either the ambulacra or interambulacra, or in 

 both. The apical disc consists of no more than the normal 

 ten plates, and the anus is placed in its centre. The 

 ambulacral areas are continuous from pole to pole. 



In the genus Palcecliinus (fig. 129) the test is spheroidal, 

 and its plates abut against one another without any over- 



