334 THE FOSSIL ECHINOIDEA 



where four come together. At the ambitus the tubercles are more crowded and the 

 intermediate spaces much narrower; on the median portion of the actinal surface, and 

 round the peristome, the tubercles are more widely spaced and the scrobicules slightly 

 larger. 



The peristome is rather large, distinctly pentagonal, with the transverse diameter 

 rather greater than the longitudinal. In small specimens the disparity of these dimen- 

 sions is greater than in fully grown tests. The ambulacral areas are slightly sunken as 

 they approach the peristome, and the bourrelets. are weU defined, especially the anterior 

 pair. This pair presents a distinctly convex margin into the peristome. The posterior 

 pair of bourrelets are the smallest, and their margins have a well-defined thickening ; 

 the odd posterior is the broadest and is the least tumid, though its margin is distinctly 

 thickened. The peristomial wall is vertical and high, and its surface is covered with 

 well-spaced small miliary granules, and these are also extended over the tumid portions 

 of the bourrelets. The peristomial margins of the ambulacral areas form small and 

 graceful concavities in the general peristomial margin, constituting the rounded angles 

 in the outline of this orifice. The innermost pair of ambulacral pores (buccal pores) 

 are weU defined, and are situated immediately over the margin, opening into the 

 peristomial wall. The phyllodes are well developed and have been described above. 



The periproct is small, transverse, with an outline varying between suboval and 

 subtriangular, placed close to the margin on the slight rounding which unites the 

 actinal surface with the ambitus, and immediately below the posterior rostration. 

 Consequent on this slightly oblique position (in relation to the plane of the actinal 

 surface), the presence of the orifice is discernible when the posterior part of the test is 

 placed on a resting plane held in the direct line of view, with the posterior extremity 

 towards the observer. This aperture has a major diameter of 6*5 millim., and a minor 

 diameter of 4 miUim. in a specimen 55 millim. in length. The major diameter of the 

 peristome is 5 millim., and the minor 3-75 millim., in a specimen of the same size. 



Young forms. Although this species is a very variable one, certain marked features 

 may be clearly traced throughout a wide' range of different growth-stages ; the modifi- 

 cations dependent on age, however, are at the same time striking and noteworthy. 



In tests measuring 38-40 millim. in length (Figs. 8-10) it will be noticed that the 

 marginal outline is more circular, that the breadth is greater, that the margins are 

 more tumid, and that the abactinal surface is more regularly convex, than in the larger 

 forms above described. It will also be seen that the test is fuller and more rounded in 

 front, less rostrate posteriorly, and that the prominent tumidity in the posterior column 

 of the postero-lateral interradia though present is much less developed ; in a rather 

 smaller test (Fig. 13), this tumidity is moved forwards to some extent, and thus 

 produces, in conjunction with the other test modifications, a marginal contour 

 approaching very nearly to a circular form. In examples of this size, the actinal 

 surface is either subplane and gradually sloping off to the very tumid margins, or even 

 faintly convex, excepting a feeble central depression around the peristome, and this 

 concavity is perhaps not always present. Indeed there appears, without question, to be 

 considerable diversity in this particular in different specimens of the same size. The 



