338 ECHINOBRISSUS 



narrow, and spear-shaped, extending from the apical disc to the posterior border; 

 ambulacral areas narrowly lanceolate ; posterior lobes short, and truncated ; base flat ; 

 inter-ambulacral areas slightly inflated at the base; single inter-ambulacrum scarcely 

 produced ; mouth-opening pentagonal, excentral ; apical disc small, central. 



Dimensions. — Height, six tenths of an inch ; antero-posterior diameter, one inch and 

 one tenth ; transverse diameter, one inch and one fifth ; the larger specimens are so much 

 deformed that their proportional dimensions cannot be accurately given. 



Description. — This species was formerly considered a variety of E. orbicularis, Phil., 

 but a careful study of many specimens of the former, compared with good typical examples 

 of the latter, convinces me that these two urchins are specifically distinct. 



The test is thin, and not often well preserved ; the specimen drawn (fig. 2 a, b) is a small 

 but tolerably perfect individual ; the outline is sub-quadrate, the shell is one tenth of an inch 

 more in breadth than in length ; it is a little narrower anteriorly, than posteriorly, and 

 the posterior border is broadly truncated; the sides are very tumid (fig. 2 c), sometimes 

 irregularly so ; and the test is higher across the apices of the postero-lateral ambulacra 

 than at any other point (fig. 2 c) ; the tumidity of the sides produces a greater flatness of the 

 upper surface than is observed in any of its congeners. The ambulacral areas are 

 nearly all of the same width; they have a narrow, graceful, lanceolate form (fig. 2 a), 

 from the mouth to about midway between the margin and the apical disc they are 

 nearly all of equal width ; at this point the pores gradually change their form, they are 

 slightly separated for a short distance, and again converge as they approach the disc; 

 the pores of the inner row are round (fig. 2 d), those of the outer row form oblique slits ; 

 the widest part of which is external ; the round pores are formed by notches in the upper part 

 and sides of the small plates forming the avenues, and the oblique pores by uncalcified 

 portions of the margins of the same plates ; from the termination of the petaloidal 

 portion of the zones to the mouth, the pairs of pores are small, and set wider apart, 

 whilst the diameter of the areas remains the same ; near the peristome they are crowded 

 close together and form arches, the convexity of which is towards the mouth-opening. 

 The inter-ambulacral areas are of unequal width ; the anterior pair are the narrowest, the 

 posterior are wider than the anterior pair, and the posterior single inter-ambulacrum is the 

 widest (fig. 2 a). The anal valley forms a long narrow depression, extending from the disc to 

 the border ; it has perpendicular sides, and a small vent opens into it about the middle ; 

 the base is flat, becoming more or less concave near the mouth (fig, 2 b) ; the anterior and 

 posterior pairs of inter-ambulacra are moderately convex in this region, and the basal 

 portion of the inter-ambulacrum is very slightly produced ; the mouth-opening is excentral, 

 situated in a depression nearer the anterior than the posterior border ; the peristome has a 

 pentagonal shape, with five rudimentary lobes. The surface of the plates is covered with 

 microscopic tubercles (fig. 2 ^), arranged in tolerably regular oblique rows ; on each plate 



