168 THE FOSSIL ECHINOIDEA 



dinal line being as 2 : 3 approximately. Seen in longitudinal profile, the anterior 

 slope of the dorsal outline is more rapid than the posterior, the degree of curvature is 

 small, and the anterior margin is thick, tumid, and well rounded ; hence it follows that, 

 when viewed from above, the anterior part of the test has a comparatively even, rather 

 than a tumid, character. The posterior slope of the longitudinal profile of the dorsal 

 surface is a gentle curve, increasing as it reaches the posterior extremity, the character 

 of the posterior rostration being scarcely perceptible in this aspect. The transverse 

 profile presents a regular convex curve on the dorsal portion, which passes gently over 

 the well-rounded margins. The actinal surface has a subconcave appearance ; the 

 peristome is sunken in a general depression of the test, and the surrounding portions 

 are slightly tumid, and pass uninterruptedly and without any rapid change of curve or 

 angularity into the tumid margins of the test. 



The ambulacral petals are long, narrow, and gracefully flexuous, expanding slightly 

 as they proceed outward, the widest part of the interporiferous area being at the extre- 

 mity. The posterior petals are the longest, and are rather broader than the others, the 

 odd anterior petal being the narrowest. The greatest width of one of the posterior petals 

 is very near to the extremity, and is about equal to one tenth of the length of the test. 

 The disparity in the length of the poriferous zones is small. The posterior petals at a 

 short distance from the apex bend slightly outward, encroaching on the lateral inter- 

 radia, the curvature then turns slightly inwards on to the posterior interradium, 

 and is again directed outwards before passing on to the rounding of the margin ; in 

 this manner a graceful flexure is produced. The anterior pair of petals curve con- 

 siderably on to the postero-lateral interradia, then over on the antero-lateral interradia, 

 and finally bend again rather sharply backwards near the extreme tips at the com- 

 mencement of the rounding of the margin ; the curvature of the posterior zone of 

 the petal being greater than the anterior. The odd anterior petal is straight. The 

 breadth of the poriferous zone diminishes rapidly at the apical, and less rapidly at the 

 outer, extremity of the zone. At the widest portion of the petal the breadth of the 

 poriferous zone is about half the breadth of the interporiferous area, whilst rather 

 nearer the apex the width of the poriferous zone is somewhat greater than this. The 

 inner pores of a zone are round, and the outer pores more or less elongate, the pair 

 being united by a deep groove. The divisional septa are narrow, and ornamented by 

 a single row of granules. The extrapetalous portion of the poriferous zone is not 

 sunken in a groove ; and the zones of each ambulacrum on the actinal surface converge 

 rapidly in the neighbourhood of the peristome. The interporiferous areas are flush 

 with the surface of the test, both within and without the petal, and their ornamenta- 

 tion is similar to that of the interradial areas. 



The anterior interradial areas have a small degree of convexity, and are not 

 disturbed by any special tumidity whatever. In the posterior part of the lateral inter- 

 radia a slight tumidity is developed, which culminates towards the margin and promi- 

 nently affects the marginal contour. The odd posterior interradium is largely tumid 

 along the median line, and especially towards the posterior extremity, where it becomes 

 well rounded and almost bombous, forming a conspicuous rostration. On the actinal 



