PALECHINOIDEA. 



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Order I. Palechinoidea. 



This order comprises all those Urchins in which the test is made up 

 of more or (rarely) fezver than twenty meridional rows of plates : while 

 the plates of the apical disc are perforated by two or more pores each, 

 or may be in part imperforate. As a general rule, there is an increase 

 in the number of rows of plates forming the test of the Palechinoids, 

 as compared with that of the Euechinoids, the interambulacral areas 

 usually consisting of more than two rows of plates each (fig. 244). 

 In some cases, the ambulacral as well as the interambulacral areas 

 may consist of more than two rows of plates each, as, for example, 



Fig. 244. — Palcechinus ellipticus. The left-hand figure shows a portion of an ambulacral area 

 enlarged. The right-hand figure exhibits a single interambulacral plate. 



in the genus Melonites (fig. 247). On the other hand, in the genus 

 Bothriocidaris there is a reduction instead of a multiplication of the 

 normal number of rows of plates in the test, the interambulacral 

 areas consisting of one row of plates only. Furthermore, in many 

 of the Palechinoids the test becomes more or less flexible, owing to 

 the fact that the plates are not suturally united by their margins, but 

 overlap by means of bevelled edges, the structure of the shell thus 

 resembling that seen in the existing group of the Echinothuridce. In 

 all the Palechinoids there is a large peristomial aperture, and a well- 

 developed masticatory apparatus (" Aristotle's lantern ") was present. 

 In all except Echinocystites (Cystocidaris), the test is "regular," the 

 anus being placed at the summit of the shell and surrounded by the 

 apical disc. The genital plates are perforated by from three to five 

 pores each ; and the ocular plates have usually two pores each, but 

 are in other cases imperforate (fig. 244). 



The order of the Palechinoids includes the three sub-orders of the 

 Cystocidaridce, Bothriocidaridce, and Perischoechinidcz, all of which 

 are wholly confined to the Palaeozoic period, with the exception of 

 the last, which is represented in the Trias by the imperfectly known 

 genus Anaulocidaris. It is interesting to note that this ancient 



