250 ACROSALENIA. 



Test sub-pentagonal, depressed on the upper surface, concave at the base ; ambulacral 

 areas convex and prominent, the anterior and posterior pairs slightly sinuous, with tw^o 

 rows of small marginal tubercles, and the intermediate space tilled with very small close-set 

 granules ; inter-ambulacral areas with from eight to ten tubercles in each row, the four 

 equatorial tubercles very large, those on the upper surface very small ; apical disc large, 

 sur-anal plate composed of eight pieces; vent elongated and extremely excentral; base 

 concave ; mouth large ; peristome equally decagonal ; primary spines long, smooth, slender, 

 and tapering ; secondary spines small, hair-like. 



Dimensions. — Height, four tenths of an inch ; transverse diameter, nine tenths of an 

 inch. 



Description. — This elegant little urchin is remarkable among its congeners for its 

 pentagonal form, arising from the flatness of the inter-ambulacral areas and the convexity 

 and prominence of the ambulacral, and likewise for exhibiting in a most remarkable 

 manner the bilateral symmetry of the Saleniad^ (fig. 1 a). 



The ambulacral areas are one fourth the width of the inter-ambulacral ; the anterior 

 single area is quite straight, and the anterior and posterior pairs are sometimes slightly 

 curved (fig. 1 a) ; the apices of the anterior pair curve gently backwards, and those of the 

 posterior pair upwards and inwards (fig. 1 a) ; two rows of small secondary tubercles, from 

 twenty to twenty-four in each row, occupy alternately the margins of the area (fig. I d), 

 those at the base and circumference being much larger (fig. 1 e. It) than those on the upper 

 surface (fig. \ d) ; the intermediate space is occupied with four rows of very small close-set 

 granules (fig. 1. d, e) ; both the tubercles and granules are extremely regular in their size 

 and arrangement throughout the areas. 



The poriferous zones are narrow and depressed, which increases the prominence of the 

 ambulacra ; the pores are unigeminal throughout, except just at the base, where they fall 

 into indistinct ranks of threes (fig. \ b, h) ; there are from eight to nine pairs of pores 

 opposite each large tubercular plate (fig. 1 e) ; and the septa are slightly elevated on the 

 surface. 



The inter-ambulacral areas are four times as wide as the ambulacral (fig. 1 c, e), they 

 are so much flattened that they form nearly straight lines at the circumference ; each 

 segment is composed of two rows of primary tubercles, about eleven in each row (fig. 1 c), 

 which are unequally developed in different regions of the area ; the four basal pairs (fig. 1 b) 

 are small, the three equatorial pairs large (fig. 1 c), and the four dorsal pairs dwarfed and 

 rudimentary (fig. 1 d, e); from the first to the eighth pair, the areolas are oval and confluent 

 (fig. 1 e), whilst those on the upper surface are surrounded by clusters of granules (fig. 1 r/) ; 

 the mihary zone is broad, and composed at the equator of six rows of small close-set 

 granules (fig. \ e) ; on the upper surface the granules cover all the surface of the plates, the 

 eight rudimentary tubercles appearing as only larger granules in their midst (fig. 1 c, d); 



