FROM THE INFERIOR OOLITE. 265 



Professor Forbes has given most beautiful figures, with full details, of Holectypua 

 hemispharicus, in Decade III, pi. 6, which leave nothing more to be desired. 



The general outline of this urchin is sub-hemispherical, but it is more or less 

 convex in different individuals; some varieties are depressed, and others are conoidal, 

 but few specimens are regularly convex ; most commonly they have a slight obliquity, 

 from the test in a majority of specimens being slightly elongated in the antero-posterior 

 diameter, and declining on the side towards the vent ; the vertex is therefore not quite 

 central, and the apical disc is nearer the anterior than the posterior border ; the sides 

 are a little tumid, and the margin is gently rounded thereby. 



The ambulacral areas are about one third the width of the inter-ambulacral ; from 

 the border to the disc they are quite conical, and taper gradually between these two 

 points ; there are six rows of tubercles at the margin, which gradually diminish to four 

 and two on the upper surface ; the two outer rows alone extend from the peristome to 

 the disc ; each pair of the small, narrow ambulacral plates (fig. 2 d) supports one tubercle, 

 which occupies the same relative position thereon on every fourth plate, so that the 

 areas are adorned at their widest part with six rows of tubercles, arranged obliquely 



in V-shaped lines, thus — • \ • * \ • In a specimen one inch and a quarter in dia- 

 meter I counted one hundred and twenty plates in each column. The poriferous zones 

 are narrow, the pores are strictly unigeminal, and there are from four to five pairs of pores 

 opposite each inter-ambulacral plate. 



The inter-ambulacral areas are rather more than three times the width of the 

 ambulacral (fig. 2 a) ; the number of plates in each column varies with the age of the 

 urchin ; in the one before me there are thirty-two plates ; those on the sides are slightly 

 bent upwards in the middle, whilst the basal plates are nearly straight; each plate 

 supports one or two tiers of tubercles, the number and arrangement of which varies 

 exceedingly in different individuals ; fig. 2 d shows a common distribution of the 

 tubercles on the plates near the margin ; besides these spinigerous tubercles, the entire 

 surface of the plates is covered with a fine, close-set, miliary granulation, from the midst 

 of which the tubercles appear to arise ; near the margin, in one specimen there are 

 sixteen tubercles abreast in one inter-ambulacral area ; the tubercles become crowded 

 towards the margin ; they increase in size at the base (fig. 2 h), but the number on the 

 plates in this region is inconsiderable. The perforated tubercles are raised on crenulated 

 bosses, which are surrounded by a sunken areolas ; at the base many of these are 

 encircled by perforated granules (fig. 2 i). 



The apical disc is small (fig. 2 a), and formed of five genital, and five ocular 

 plates ; the pairs of genital plates are shield-shaped and perforated ; the single plate is 

 imperforate ; the right anterior genital plate is the largest, and extends into the centre of 

 the disc, supporting on its surface the spongy madreporiform body. The ocular plates 



