216 STOMECHINUS. 



elevated, and very regularly formed ; its surface is divided into fifteen nearly equal-sized 

 lobes grouped in five divisions, with three lobes in each, of which the ambulacra form the 

 centre, and the half of the adjoining inter-ambulacra the two lateral lobes (fig. 4 a). 



The distinctive character of this species consists in the wide, naked median depression 

 in the centre of the inter-ambulacral areas, which extends from the circumference to the 

 apical disc, and is throughout entirely destitute of granules or any other sculpture 

 (fig. 4 a) ; near the circumference, however, small tubercles occupy the space, and at this 

 point there are twelve tubercles, in a horizontal row, in a single inter-ambulacral area ; each 

 area is thus divided into two convex lobes by a median sulcus ; in each lobe there is one 

 complete principal row of twenty-five tubercles, which extends from the peristome to the 

 disc, and two incomplete rows on each side of the principal row, with from twelve to 

 eighteen tubercles in each, which disappear on the sides ; besides these at the widest part 

 of the area, a few small additional tubercles are introduced (fig. 4 c, d); the bosses are 

 small, and closely surrounded by circles of granules (fig. 4^/); the tubercles are large and 

 prominent, and nearly all of the same size, which gives the surface of this urchin very 

 much the appearance of a large Polycyplius. 



The ambulacral areas are nearly half as wide as the inter-ambulacral ; they are 

 furnished with two complete rows of tubercles disposed on the margins of the area, and 

 two incomplete rows which occupy the central parts of the sides (fig. 4d); the tubercles 

 have small bosses, around which granules are disposed in circles, and similar moniliform 

 granular rings surround the larger tubercles of the incomplete rows (fig. 4 d). 



The tubercles of both areas are large, prominent, and highly polished ; those at the 

 base are a little larger than the tubercles on the sides ; and it may be said of this species in 

 general that the test is uniformly very granular, and forms a remarkable contrast to that of 

 Stomechinus hi granulans (fig. 3 a). 



The poriferous zones are narrow, the trigeminal ranks form angles of about 65°, which 

 become nearly sub-parallel in the upper part of the zone and at the base (fig. 4 b, c); two 

 small granules are dotted between each file of pores. 



The apical disc is small and central (fig. 4 a); the genital plates are nearly equal-sized, 

 with the exception of the right anterolateral, which is the largest, and supports a fine 

 spongy madreporiform body (fig. 4e); a series of granules on the plates form a circle 

 around the vent, which is of a moderate size. The ocular plates are small, pentagonal 

 pieces, projecting from between the angles of the genitals ; there are two holes in each 

 plate, with a transverse slit between (fig. 4 e). The oviductal holes are conspicuous, and 

 perforated near the outer third of the plate. 



The mouth opening is very large, nearly one half the diameter of the test (fig. 4 b); the 

 peristome is pentagonal, with two wide, obtuse notches at each angle, and a small, lip-like 

 lobe between ; the ambulacra! lobes are four times as broad as the inter-ambulacral, and 

 form the sides of the pentagon. 



