HEAKT-SHAPED UKCHINS. 



191 



this simple form, to assume a more or less ilecided heart- 

 shape, with one or both apertures of the test becoming 

 eccentric, and -with a frecpient tendency for the perforated 

 portions of the ambulacral areas to become restricted to 

 the central part of the upjier surface, where they form 

 a flower-like pattern (Fig. 60). Such types constitute 

 the group of irregular urchins, which, on the d<3Ctrine of 

 evolution, may be safely regarded as derived from the 

 regular grouj). Moreover, not satisfied with the assumjj- 

 tion of these new sliapes, the irregular urchins appear to 

 have considered the " Aristotle's lantern," which had 

 served their ancestors as a masticating organ for countless 

 ages, only a useless in- 

 cumbrance, and, accord- 

 ingly, the more advanced 

 "radicals" among them 

 have totally discarded this 

 piece of apparatus, and, 

 indeed, appear to get on 

 equally well without it. 



The forms connecting 

 the more aberrant mem- 

 bers of the irregular with 

 the regular group of 

 urchins are both numerous 

 and varied. Among them 

 we may refer to the com- 

 mon little helmet urchin 

 (Echinoconus) of our 

 chalk. The test of this 

 species is about an inch 

 in height, and forms a tall 

 cone, which is not quite radiately symmetrical. The 

 mouth, indeed, retains its usual position at the basal 

 pole, while the perforated areas extend from pole to pole. 

 In place, however, of the vent being situated in the centre 

 of the apical disc, it has been transferred to the margin of 

 the lower surface, in the middle of one of the intermediate 

 areas. It is thus placed opposite to an ambulacral area 

 corresponding to the one directed upwards in Fig. 57 ; 



Fig. 60. — Upper surface of tlie Test 

 of a Cretaceous Heart - Urchin 

 (Micraster). a, ambulacral areas ; 

 (■ a intermediate areas. 



