236 ACROSALENIA. 



the upper ; they exhibit a very slight undulating contour, and have two rows of small, 

 nearly equal-sized secondary tubercles, from. sixteen to eighteen in each row (fig. 4 c) ; the 

 tubercles at the basal angle are the largest (fig. 4 d, e), and they imperceptibly diminish in 

 size from the circumference to the disc (fig. 4 a), they are all perforated and raised on 

 bosses, which have ten crenulations on their summits (fig. 4< d); the tubercles of each row 

 alternate, and in the centre of the area there are one or two rows of granules, which send 

 ofi" small lateral branches to encircle the areolas (fig. 4 d, e). 



The inter-ambulacral areas are nearly four times as wide as the ambulacral ; they consist 

 of two columns, each composed of seven or eight plates, each plate bearing a primary 

 tubercle (fig. 4i d) ; the two plates nearest the peristome are very small (fig. 4 6), the four on 

 the sides are very large (fig. 4 c), and the two near the disc are small (fig. 4 a) ; the 

 tubercles are elevated on very large bosses (fig. 4 /), which have ten or more deep 

 crenulations on their summits, the tubercles themselves are perforated ; around the 

 base of the boss there is a wide, smooth areola ; the plates are bordered by a single row 

 of granules (fig. 4i d), which, however, is sometimes absent from the upper and lower 

 borders, the areolas then being confluent ; the miliary zone is narrow, and is formed of two 

 rows of granules, with the intermediate angles having a few smaller additional granules 

 introduced ; the areolas are separated from the poriferous zones by a row of granules on the 

 zonal side of the plates (fig. 4 c? ) , 



The poriferous zones are narrow, the pores unigeminal throughout, except at 

 the base, where they fall into triple oblique rows; the septa form small elevations 

 on the surface, and a beaded line thereby passes down the zone between the pores 

 forming a pair (fig. 4 d), and there are nine or ten pairs of pores opposite each tubercular 

 plate. 



The apical disc is often admirably preserved in this beautiful urchin ; the study of its 

 curious structure first enabled me to correct M. Agassiz's erroneous supposition, that 

 the genital plate, which carries the madreporiform body, is the single plate, and repre- 

 sents the posterior side of the animal, instead of the right antero-lateral plate which 

 occupies the same relative position in all the Echinoidea. 



The disc is one third the width of the test, and in consequence of the projection of the 

 genital plates has a pentagonal form ; it is shghtly convex and prominent, the anterior 

 and posterior pair of genital plates are nearly of the same size (fig. 4 a, i), the right 

 anterior is the largest, and carries on its front part the madreporiform body ; the small 

 crescentic-shaped single plate forms the posterior boundary of the vent (fig. 4 i) ; the 

 oviductal holes are all near the apices; the sur-anal plate is composed of six or seven 

 pieces arranged like mosaic before the anal opening ; the ocular plates are small and 

 firmly wedged between the genital and sur-anal elements. All the plates of the apical disc 

 have numerous granules scattered on their surface. 



The mouth opening is wide, nearly one half the diameter of the test ; the peristome is 

 decagonal ; the ambulacral being larger than the inter-ambulacral lobes, the ten deep 



