168 HEMIPEDINA. 



desired that a more diligent search should be made for Echinoderms, in Kimmeridge Clay- 

 districts, as the little at present known of this class consists only of fragments of tests 

 and detached spines. 



The ambulacral areas are narrow, with two marginal rows of small tubercles, rather 

 irregular in the mode of their arrangement (fig. 3 b) ; the poriferous zones are straight, 

 the pores are placed obliquely, and there are four pair of pores opposite each large plate. 



The inter-ambulacral areas are three times the width of the ambulacral ; they have 

 only one row of tubercles on the zonal sides of the plates (fig. 3 h), which leaves a 

 wide space between the areolas and the central suture ; this is filled with four or five 

 rows of small granules, which, with those on the adjoining plate, form a miliary zone 

 with from eight to ten rows of granules ; the bosses are large and prominent, and 

 the tubercles are of proportionate magnitude, the areolas are complete, and surrounded by 

 circles of granules of the same size as those which fill the zone. 



Affinities and differences. — The fragment (fig. 3 a) formed part of a species belonging 

 to the first section of the genus, with two rows of tubercles, and a wide miliary zone in 

 the inter-ambulacra, but, like the preceeding species, it is too imperfect for comparison. 



Locality and Stratigrajohical position. — This fragment was collected from the 

 Kimmeridge Clay, near Aylesbury, by Z. Hunt, Esq. and was kindly communicated by 

 Professor Morris. 



