FROM THE LIAS. 147 



tubercles, which extend from the peristome to the disc ; inter-ambulacral areas with two 

 rows of tubercles set near the poriferous zones, eleven to twelve in each row ; a delicate 

 circle of small granules around each, and a naked space in the centre of the miliary zone ; 

 mouth opening small, peristome decagonal; base exhibiting very regular radii of tuber- 

 cular rows. 



Dimensions. — Height, one fifth of an inch ; transverse diameter, half an inch. 



Description. — The test of this beautiful little Diadem is small, circular, and depressed 

 on the upper and under surfaces ; the ambulacral areas are narrow, with two rows of small 

 tubercles (fig. 4 d) set alternately on the sides of the area, close to the poriferous zones 

 (fig. 4 g) ; a line of granules occupies the course of the central suture, and additional 

 granules fill up the space between that line and the pores (fig. 4 g) ; there are from twelve 

 to fourteen tubercles in each row ; about one third the size of those of the inter-ambulacra, 

 but much smaller in proportion in the upper part of the area (fig. 4 d) ; the zones are 

 narrow and straight throughout ; the septa forming slight papillae on their surface ; the pores 

 are very small, and there are four pairs of holes opposite each of the large plates. 



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

 have two rows of primary tubercles, about ten or eleven in each row ; the tubercles occupy 

 the centre of the plates (fig. 4 g) ; the boss is prominent, the tubercle small, and the areola 

 not well defined ; on each side of the tubercle there are two rows of granules, the inner of 

 which forms a circle around the boss (fig. 4 g) ; the miliary zone is wide (fig. 4/), and 

 filled with the granules just described ; interspersed among them are several which pass 

 into the condition of minute tubercles, being raised on miniature bosses, and having their 

 surface perforated. On the upper part of the area the granules disappear from the centre, 

 leaving a portion of the plates naked (fig. 4</); as the granules are disposed with much 

 regularity, the test of this species has a highly ornamented appearance (fig. 4 d, e,f). 



The base is concave, and the mouth opening, which is less than half the diameter of 

 the shell, is situated in a depression ; the peristome is decagonal, the ambulacral being 

 larger than the inter-ambulacral lobes : as the primary tubercles of the inter-ambulacra are 

 set close together at the base, they form prominent tuberculated radii (fig. 4 e) in this 



region. 



The discal opening is small, but the plates are absent. 



Affinities and differences. — This species resembles H. Etheridgii, but it is a smaller 

 and more depressed form ; the tubercles of the ambulacra are larger in size and fewer in 

 number ; and those of the inter-ambulacra want the well-defined areolas around them, 

 seen in the primary tubercles of H. Etheridgii (fig. 5 g). The discal opening is likewise 

 a much smaller aperture in H. Jardinii than in //. Etheridgii. 



