268 PALEONTOLOGY 



able. The three anterior ambs can be traced directly 

 down to the mouth ; the two posterior extend to the 

 posterior end of the plastron, whence they can be traced 

 with some difficulty along either side of the plastron. It 

 is evident that the abundant tube-feet of the dorsal surface 

 cannot serve to convey food to the mouth, since they are 

 separated from it by a wide space with very scanty tube- 

 feet : probably they serve as gills. These aboral regions 

 of abundant pore-pairs are called the petals, from a 

 resemblance to an open five-petalled flower ; but Micraster 

 is not typically " petaloid," only sub-petaloid, because the 

 two series of pores in each petal are nearly parallel, 

 instead of widening out and closing in again. 



The interamb plates, having to occupy all the space 

 between the ambs, are for the most part very large. 

 Their regular double-row becomes rather confused on 

 the under side. The posterior interamb on the under 

 side constitutes the plastron. 



All plates except those of the petals bear tubercles and 

 granules. On the upper surface the tubercles are small, 

 not very prominent, and scattered, with abundant small 

 granules between. On the under surface they are larger, 

 with more deeply excavated areola, and are much more 

 closely set ; the granules, though fewer in number, being 

 also coarser and more close-set. Running round the 

 lower corner of the vertical posterior end there is a 

 narrow ring which at first sight appears smooth, but 

 under magnification is seen to be covered with extremely 

 fine granules. This is a fascicle, and from its position is 

 called a sub-anal fascicle. 



