262 HOLECTYPUS. 



inter-ambulacrum slightly produced and truncated (fig. \a,b). The ambulacral areas 

 are one third the width of the inter-ambulacral ; they have a lanceolate form, and 

 taper gradually from the border to the disc ; at the circumference there are 

 eight rows of tubercles, which are disposed so as to form double oblique 



. • • • 



rows, thus — • \ '. . . '. \ ' (fig. 1 d). In the upper part of the areas the inner 



rows disappear, and the test is depressed in the line of the median suture (fig. 1 a). 



The inter-ambulacral areas are three times as wide as the ambulacral, each plate 

 supports a number of tubercles, one row represents the principal range, and extends from 

 the peristome to the disc, and the tubercles of this row are a little larger than the others 

 on the upper surface, but nearly of the same size on the sides and base. Between 

 this primary row and the centro-suture, the tubercles are arranged in a single line 

 on the same plane ; but between it and the poriferous zones they form three super- 

 imposed rows (fig. 1 d), so that the zonal side of the plates contains many more tubercles 

 than the central half. The tubercles are surrounded by sunken areolas ; they have 

 crenulated bosses and perforated summits, and the intermediate surface of the plates is 

 covered with a fine, abundant, miliary granulation. The tubercles at the base are much 

 larger, they have deeper areolas, and are all disposed in single, concentric rows (fig. 1 b). 



The poriferous zones are extremely narrow, and the pores are unigeminal throughout. 

 The septa are elevated above the surface, and form a moniliform line between the pores 

 (fig. Id), of which there are eight pairs opposite each inter-ambulacral plate. 



The apical disc is well preserved in many of our specimens. Pig. 1 i, is an accurate 

 drawing of this part (fig. I a). The anterior and posterior pairs of genital plates 

 are perforated, but the single posterior plate is imperforate ; the antero-lateral plate 

 is very large, and extends into the centre of the disc ; it supports a prominent, spongy, 

 madreporiform body, which forms the summit of the test, and occupies the centre of 

 the disc ; not, however, as a new element introduced into the centre of the genital circle, 

 but formed simply by the development of the right antero-lateral plate. The ocular plates 

 are small, heart-shaped bodies, inserted between the genitals (fig. 1 ^), with the eye-holes 

 perforated near their margin (fig. 1 i) ; all the discal plates are covered with small 

 tubercles, similar to those which form the miliary granulation. In M. Desor's 'Monograph 

 on the Galerites,' tab. 10, fig. la, the eye-holes are represented as marginal, and the 

 madreporiform body is figured and described as distinct from the genital plates. 

 It is probable that the imperfection of his specimens led the learned author into these 

 errors, which we have now the pleasure of correcting by our better examples. The under 

 surface is slightly concave, the mouth opening is central, and upwards of one third 

 the diameter of the test (fig. 1 b). The peristome is nearly equally decagonal, and 

 the notches are wide, with reflected margins. In one specimen, collected by Mr. William 

 Buy, from the Forest Marble of Wiltshire, the jaws are preserved in situ (fig. ly). 



