192 SEA-UECHINS. 



and we thus, for the first time, find a regular bilateral 

 symmetry fully establislied. In the particular species to 

 which we have referred (as in all the under-mentioned 

 forms), the "Aristotle's lantern " has been lost, although 

 it is retained in other members of the same group. One 

 specimen of a helmet urchin lias been discovei'ed showing 

 the rare abnormality of having but four ambulacral areas. 



A further step is shown by the so-called sugar-loaf 

 urchin (Aiianchytes) of the English chalk, the tall silicified 

 tests of which are so commonly found in the gravel derived 

 from the denudation of the chalk, in this species, which 

 is considerably larger and more ovoid than the helniet 

 urchin, the elevated form of the test is still retained, but 

 the contour of its under surface has become more decidedly 

 heart-shaped. JNloreover, the vent has become shifted 

 completely on to the lower surface ; while the mouth, 

 although remaining (as is invariably the case) on the 

 under surface, has travelled away from its original central 

 position so as to be jilaced in the ambulacral area corre- 

 sponding to the one directed upwards in Fig. 57. Here, 

 therefore, we have the bilateral symmetry still more 

 distinctly marked. Another advance shown in this urchin 

 is the circumstance that the perforated portions of the 

 ambulacral areas, instead of extending on to the lower 

 surface of the test, stop short at the edge. Some members 

 of the same family are still more peculiar in that the apical 

 disc is gi-eatly elongated from front to back, in consequence 

 of which the five ambulacral areas do not all meet one 

 another at the summit of the test : those corresponding to 

 the three turned away from the spectator in Fig. 57 

 meeting near the middle of the test; while the remaining 

 two are brought together in the opposite jsart of the test 

 corresponding to the lower portion of the same figure. 



A third common chalk species, the heart-urchin, or 

 fairy heart of the quarrymen (MicV'/ster), affords a good 

 example of what may be regarded as the extreme modifi- 

 cation cif structure developed in the group. The upper 

 surface of one of these urchins is represented in Fig. 60, 

 from which it will he seen that the contour is regularly 

 heart-shaped, and the whole test much depressed. The 



