132 



ECHINODERMA. ECHINOIDEA 



Micraster (figs. 50, 53). Test heart-shaped or oval. Apical 

 disc small, excentric. Ambulacra sub- 

 petaloid, placed in sunken areas, the 

 sub-petaloid parts of the two an- 

 terior lateral longer than those of the 

 two posterior lateral ; pores unigemi- 

 nal. The anterior unpaired ambu- 

 lacrum in a deep groove, with its 

 pores circular. Interambulacra with 

 large plates ; tubercles small, per- 

 forate and crenulate. Fasciole below 

 the anus. Peristome near the anterior 

 border, with a projecting lip. Peri- 

 proct on the upper part of the posterior 

 end. On the under surface the pos- 

 terior interambulacrum bulges out 



forming a plastron. Middle and Upper Chalk ; also Tertiary in 

 Australia. Ex. M. cor-anguiiium, Upper Chalk. 



Hemiaster. Form similar to Micraster. A peripetalous 

 fasciole only. Cretaceous to present day. Ex. H. bailyi, Gault. 



Schizaster. Test heart-shaped, highest behind, with excentric 

 apex. Anterior ambulacrum long, placed in a groove; other ambulacra 

 petaloid and in deep grooves — the posterior pair much shorter than 

 the antero-lateral pair. Peristome near the anterior margin, with 

 projecting lip. Periproct on the posterior truncated end of the 

 test. A peripetalous fasciole, and usually also a lateral fasciole 

 diverging from the former and passing beneath the periproct. 

 Eocene to present day. Ex. S. cFurbani, Bracklesham Beds. 



^ea; 



Fig. 53. Micraster cor-bovis. 

 Upper Chalk, x \. 



Distribution of tlie Echinoidea 



Some echinoids live at great depths in the ocean, no 

 less than a dozen species having been found below the 

 2000 fathom line, and one even at 2900 fathoms ; but by 

 far the larger number occur near the coasts in shallow 

 water: thus, of the 297 existing species recorded by 

 Agassiz, 201 are found in water of less than 150 fathoms 

 in depth. Echinoids are most abundant where the sea- 



