82 ECHINODERMATA. ECHINOIDEA. 



areas large, simple ; pores unigeminal, round or elongate, 

 the anterior ambulacrum in the groove. Interambulacral 

 areas with small tubercles and granules. Peristome 

 excentric and elliptical. Jaws absent. Periproct supra- 

 marginal. Upper Greensand and Lower Chalk ; also Ter- 

 tiary in Australia. 



Micraster. Test cordiform or oval. Apical disc 

 small, excentric. Ambulacral areas subpetaloid, placed in 

 sunken areas, the subpetaloid part of the two anterior- 

 lateral longer than the two posterior-lateral ; pores 

 unigeminal. The anterior unpaired ambulacrum in a 

 deep groove, pores round. Interambulacral areas with 

 large plates ; tubercles small, perforate and crenulate. 

 Subanal fasciole. Peristome excentral, near the anterior 

 border, with a projecting lip. Jaws present. Periproct 

 supramarginal. Middle and Upper Chalk ; also Tertiary 

 in Australia. 



Distribution of the Echinoidea. 



Some Echinoids live at great depths in the ocean, one 

 species being found at 2900 fathoms, but by far the larger- 

 number occur near the coasts in shallow water. They 

 are most abundant where the sea-bottom is rocky, sandy, 

 or calcareous, and less common where it is muddy ; 

 consequently fossil forms are rare in clayey strata. Those 

 found in deep water have a much wider range in space 

 than those found in shallow water, and many of the 

 genera, especially those with a considerable range in 

 depth, have also a long range in time, some extending 

 back to the Cretaceous period and even further. 



There is a marked contrast in the distribution of the 

 two orders of the Echinoidea. The Palseechinoids are 



